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THE  LOUIS  CLARK  VANUXEM  FOUNDATION 
LECTURES  FOR  1915-1916 


®t|?  ICnuts  (ftlark  Bamixrm  iFmutoatum 
of  Prinrttott  Utiturrsttg 

was  established  in  1913  with  a  bequest  of  $35,000 
under  the  will  of  Louis  Clark  Vanuxem,  of  the 
Class  of  1879.  By  direction  of  the  executors  of 
Mr.  Vanuxem's  estate,  the  income  of  the  foun- 
dation is  to  be  used  for  a  series  of  public  lectures 
delivered  in  Princeton  annually,  at  least  one  half 
of  which  shall  be  on  subjects  of  current  scientific 
interest.  The  lectures  are  to  be  published  and 
distributed  among  schools  and  libraries  generally. 


The   following  lectures   have  already  been   pub- 
lished or  are  in  press: 

1913-13     The  Theory  of  Permutable  Functions,  by 
Vito  Volterra 

1913-14  Lectures  delivered  in  connection  with  the 
dedication  of  the  Graduate  College  of 
Princeton  University  by  Emile  Boutroux, 
Alois  Riehl,  A.  D.  Godley,  and  Arthur 
Shipley 

1914-15     Romance,   by    Sir    Walter    Raleigh 

1915-16  A  Critique  of  the  Theory  of  Evolu- 
tion, by  Thomas  Hunt  Morgan 


LOUIS  CLARK  VANUXEM  FOUNDATION 

A  CRITIQUE 

OF  THE 

THEORY  OF  EVOLUTION 


BY 

THOMAS  HUNT  MORGAN 

PROFESSOR  OF  EXPERIMENTAL  ZOOLOGY  TN 
COLUMBIA  UNIVERSITY 


LECTURES  DELIVERED  AT  PRINCETON  UNIVERSITY 
FEBRUARY  24,  MARCH  1,  8,  15,  1916 


PRINCETON  UNIVERSITY  PRESS 

PRINCETON 

LONDON: HUMPHREY  MILFORD 

OXFORD  UNIVERSITY  PRESS 

1916 


Copyright,  1916,  by 
Princeton  University  Press 

Published  October.  1916 


PREFACE 

Occasionally  one  hears  today  the  statement 
that  we  have  come  to  realize  that  we  know  noth- 
ing about  evolution.  This  point  of  view  is  a 
healthy  reaction  to  the  over-confident  belief 
that  we  knew  everything  about  evolution. 
There  are  even  those  rash  enough  to  think  that 
in  the  last  few  years  we  have  learned  more 
about  evolution  than  we  might  have  hoped  to 
know  a  few  years  ago.  A  critique  therefore 
not  only  becomes  a  criticism  of  the  older  evi- 
dence but  an  appreciation  of  the  new  evidence. 

In  the  first  lecture  an  attempt  is  made  to  put 
a  new  valuation  on  the  traditional  evidence  for 
evolution.  In  the  second  lecture  the  most  re- 
cent work  on  heredity  is  dealt  with,  for  only 

characters  that  are  inherited  can  become  a  part 

v 


vi  PREFACE 

of  the  evolutionary  process.  In  the  third  lec- 
ture the  physical  basis  of  heredity  and  the  com- 
position of  the  germ  plasm  stream  are  examined 
in  the  light  of  new  observations;  while  in  the 
fourth  lecture  the  thesis  is  developed  that 
chance  variation  combined  with  a  property  of 
living  things  to  manifold  themselves  is  the 
key  note  of  modern  evolutionary  thought. 

T.  H.  Morgan 

July,  1916 


TABLE  OF  CONTEXTS 

CHAPTER  I 

A     REVALUATION     OF     THE     EVIDENCE     ON 

WHICH  THE  THEORY  OF  EVOLUTION 

WAS  BASED 

PAGE 

Preface     v 

1.   Three  Kinds  of  Evolution 1-7 

2   The  Evidence  for  Organic  Evolution 7-27 

a.  The  Evidence  from  Comparative  Anatomy   7-14 

b.  The    Evidence    from    Embryology 14-23 

c.  The   Evidence   from    Paleontology 24-27 

3.  The  Four  Great  Historical  Speculations.  .  27-39 

a.  The  Environment 27-31 

Geoff roy  St.  Hilaire 

b.  Use    and    Disuse 31-34 

From  Lamarck  to  Wei sm ami 

c.  The  Unfolding  Principle 34-36 

Nageli   and   Bateson 

d.  Natural    Selection 36-39 

Darwin 

vii 


viii  CONTEXTS 

CHAPTER   II 

THE  BEARING  OF  MENDEL'S  DISCOVERY  ON 
THE  ORIGIN  OF  HEREDITY  CHARACTERS 

1.  Mendel's    First    Discovery — Segregation 41-52 

2.  Mendel's  Second  Discovery — Independent  As- 

sortment     52-59 

3.  The    Characters    of    Wild    Animals    and    Plants 

Follow  the  Same  Laws  of  Inheritance  as  do 
the  Characters  of  Domesticated  Animals  and 
Plants    59-8  i 

a.  Sexual  Dimorphism    61-61 

Eosin  eye  color  of  Drosophila 61-62 

Color    of   the    Clover    Butterfly,    Colias 

philodice 62-63 

Color    of    Papilio    turnus 63 

Color  pattern  of  Papilio  polytes 63-61 

b.  Duplication    of    parts 65-66 

Thorax  of  Drosophila 65 

Legs    of    Drosophila 65-66 

c.  Loss  of  characters 66-68 

"Eyeless"  of  Drosophila 66-67 

Vestigial   wings   of   Drosophila 67 

Bar  eye  of  Drosophila 67-68 

d.  Small   changes   of   characters 68-70 

"Speck" 68 

Bristles    of    "club"    70 

e.  Manifold  effects  of  same  factor 71 

f.  Constant   but   trivial    effects    may   be   the 

product    of    factors    having    other    vital 
aspects     - 73 


CONTEXTS  ix 

g.  Sex-linked    inheritance 75-80 

in   Drosophila    ampelophila 75-76 

in  the  wild  species  I),  repleta 76 

in    man    77 

in  domesticated   Fowls 77-7S 

in   the  wild  moth.  Abraxas 78-80 

h.    .Multiple  allelomorphs 81-84 

in  the  wild  Grouse  Locust 81-83 

in   domesticated    mice    and    rabbits ....  83 

in  Drosophila  ampelophila 84 

1.   Mutation    and    Evolution 84-88 

CHAPTER  III 

THE     FACTORIAL     THEORY     OF     HEREDITY 

AND  THE  COMPOSITION  OF  THE 

GERM    PLASM 

1.   The    Cellular    Basis   of   Organic    Evolution 

and  Heredity 89-9S 

c2.   The     Mechanism     of     Mendelian     Heredity 
Discovered     in     the     Behavior     of     the 

Chromosomes 98-10-2 

3.    The    Four   Great    Linkage    Groups   of    Dros- 
ophila  ampelophila    103-1 18 

a.  Group    1 101-109 

b.  Group    II 109-112 

c.  Group   III 112-115 

d.  Group    TV 115-1  IS 

1.    Localization    of    Factors    in    the    Chromo- 
somes   118-1  t'2 

a.   The    Evidence    from    Sex    Linked 

Inheritance 118-137 


x  CONTEXTS 

b.  The  Evidence  from  Interference.  .  137-138 

c.  The      Evidence      from      Non-Dis- 

junction  1  39-142 

5.  How  Many  Genetic   Factors  are  there  in 

the    Germ-Plasm    of    a     Single     Indi- 
vidual?  142-113 

6.  Conclusions    144 

CHAPTER   IV 

SELECTION   AND   EVOLCTIOX 

1.  The  Theory  of  Natural  Selection 145-161 

2.  How   has  Selection   in  Domesticated  Ani- 

mals   and    Plants    rrought    about    its 
Results? 161-16.5 

3.  Are  Factors  Changed  through  Selection?  165-187 

4.  How   does    Natural    Selection    Influence 

the    course    of    evolution? 187-193 

5.  Conclusions   193-194 

Index    .■* .  195-197 


CHAPTER  I 

A  REVALUATION  OF  THE  EVIDENCE  ON 

WHICH  THE  THEORY  OF  EVOLUTION 

WAS  BASED 

We  use  the  word  evolution  in  many  ways — to 
include  many  different  kinds  of  changes.  There 
is  hardly  any  other  scientific  term  that  is  used 
so  carelessly — to  imply  so  much,  to  mean  so 
little. 

Three  Kinds  of  Evolution 

We  speak  of  the  evolution  of  the  stars,  of 
the  evolution  of  the  horse,  of  the  evolution  of 
the  steam  engine,  as  though  they  were  all  part 
of  the  same  process.  What  have  they  in  com- 
mon ?  Only  this,  that  each  concerns  itself  with 
the  history  of  something.  When  the  astron- 
omer thinks  of  the  evolution  of  the  earth,  the 
moon,  the  sun  and  the  stars,  he  has  a  picture  of 
diffuse  matter  that  has  slowly  condensed.  With 
condensation  came  heat;  with  heat,  action  and 


2  THEORY  OF  EVOLUTION 

reaction  within  the  mass  until  the  chemical  sub- 
stances that  we  know  today  were  produced. 
This  is  the  nebular  hypothesis  of  the  astrono- 
mer. The  astronomer  explains,  or  tries  to 
explain,  how  this  evolution  took  place,  by  an 
appeal  to  the  physical  processes  that  have 
been  worked  out  in  the  laboratory,  processes 
which  he  thinks  have  existed  through  all  the 
eons  during  which  this  evolution  was  going  on 
and  which  were  its  immediate  causes. 

When  the  biologist  thinks  of  the  evolution 
of  animals  and  plants,  a  different  picture  pre- 
sents itself.  He  thinks  of  series  of  animals 
that  have  lived  in  the  past,  whose  bones  (fig. 
1 )  and  shells  have  been  preserved  in  the  rocks. 
He  thinks  of  these  animals  as  having  in  the  past 
given  birth,  through  an  unbroken  succession 
of  individuals,  to  the  living  inhabitants  of  the 
earth  today.  He  thinks  that  the  old,  simpler 
types  of  the  past  have  in  part  changed  over  into 
the  more  complex  forms  of  today. 

He  is  thinking  as  the  historian  thinks,  but 
he  sometimes  gets  confused  and  thinks  that  he 
is  explaining  evolution  when  he  is  only  describ- 
ing it. 


.'-» 


Fig.  1.     A  series  «>t'  skulls  and  feet.     Eohippus,  Mesohippus, 
Meryhippus,    Hipparion    and    Equus.      (American    Museum    of 

Natural    History.      After   Matthews.) 


4  THEORY  OF  EVOLUTION 

A  third  kind  of  evolution  is  one  for  which 
man  himself  is  responsible,  in  the  sense  that  he 
has  brought  it  about,  often  with  a  definite  end 
in  view. 

His  mind  has  worked  slowly  from  stage  to 
stage.  We  can  often  trace  the  history  of  the 
stages  through  which  his  psychic  processes 
have  passed.  The  evolution  of  the  steam-boat, 
the  steam  engine,  paintings,  clothing,  instru- 
ments of  agriculture,  of  manufacture,  or  of 
warfare  (fig.  2)  illustrates  the  history  of  hu- 
man progress.  There  is  an  obvious  and 
striking  similarity  between  the  evolution  of 
man's  inventions  and  the  evolution  of  the  shells 
of  molluscs  and  of  the  bones  of  mammals,  yet 
in  neither  case  does  a  knowledge  of  the  order 
in  which  these  things  arose  explain  them.  If 
we  appeal  to  the  psychologist  he  will  probably 
tell  us  that  human  inventions  are  either  the  re- 
sult of  happy  accidents,  that  have  led  to  an 
unforeseen,  but  discovered  use;  or  else  the  use 
of  the  invention  was  foreseen.  It  is  to  the 
latter  process  more  especially  that  the  idea  of 
purpose  is  applied.  When  we  come  to  review 
the  four  great  lines  of  evolutionary  thought  we 


THEORY  OF   EVOLUTION 


4- 


* 


"I  ,  v 


I  4-  r 


U 


A*  Mill  SjKir 

am  ri!i 


Fig.    2.      Evolution    of    pole    anus.      (Metropolitan    Museum. 
After   Dean.) 

shall  see  that  this  human  idea  of  purpose  recurs 
in  many  forms,  suggesting  that  man  has  often 
tried  to  explain  how  organic  evolution  has 
taken  place  by  an  appeal  to  the  method  which 


6  THEORY  OF  EVOLUTION 

lie  believes  he  makes  use  of  himself  in  the  in- 
organic world. 

What  has  the  evolution  of  the  stars,  of  the 
horse  and  of  human  inventions  in  common '. 
Only  this,  that  in  each  case  from  a  simple  be- 
ginning through  a  series  of  changes  something 
more  complex,  or  at  least  different,  has  come 
into  being.  To  lump  all  these  kinds  of  changes 
into  one  and  call  them  evolution  is  no 
more  than  asserting  that  you  believe  in  con- 
secutive series  of  events  (which  is  history) 
causally  connected  (which  is  science)  ;  that  is, 
that  you  believe  in  history  and  that  you  believe 
in  science.  But  let  us  not  forget  that  we  may 
have  complete  faith  in  both  without  thereby 
offering  any  explanation  of  either.  It  is  the 
business  of  science  to  find  out  specifically  what 
kinds  of  events  were  involved  when  the  stars 
evolved  in  the  sky,  when  the  horse  evolved  on 
the  earth,  and  the  steam  engine  was  evolved 
from  the  mind  of  man. 

Is  it  not  rather  an  empty  generalization  to 
say  that  any  kind  of  change  is  a  process  of  evo- 
lution? At  most  it  means  little  more  than  that 
vou  want  to  intimate  that  miraculous  interven- 


THEORY  OF  EVOLUTION  7 

tion  is  not  necessary  to  account  for  such  kinds 
of  histories. 

We  are  concerned  here  more  particularly 
with  the  biologists'  ideas  of  evolution.  My  in- 
tention is  to  review  the  evidence  on  which  the 
old  theory  rested  its  case,  in  the  light  of  some 
of  the  newer  evidence  of  recent  years. 

Four  great  branches  of  study  have  fur- 
nished the  evidence  of  organic  evolution.  They 
are: 

Comparative  anatomy. 

Embryology. 

Paleontology. 

Experimental  Breeding  or  Genetics. 

The  Evidence  from  Comparative  Anatomy 
When  we  study  animals  and  plants  we  find 
that  they  can  be  arranged  in  groups  according 
to  their  resemblances.  This  is  the  basis  of  com- 
parative anatomy,  which  is  only  an  accurate 
study  of  facts  that  are  superficially  obvious  to 
everyone. 

The  groups  are  based  not  on  a  single  differ- 
ence, but  on  a  very  large  number  of  resem- 
blances. Let  us  take  for  example  the  group  of 
vertebrates. 


8  THEORY  OF  EVOLUTION 

The  hand  and  the  arm  of  man  are  similar  to 
the  hand  and  arm  of  the  ape.  We  find  the 
same  plan  in  the  forefoot  of  the  rat,  the  ele- 
phant, the  horse  and  the  opossum.  We  can 
identify  the  same  parts  in  the  forefoot  of  the 
lizard,  the  frog  (fig.  3),  and  even,  though  less 


Fig.  3.  Limb  skeletons  of  extinct  and  living  animals,  show- 
ing the  homologous  bones:  1,  salamander;  2,  frog;  3,  turtle; 
4,  Aetosaurus;  5,  Pleisiosaurus;  6,  Ichthyosaurus;  7,  Meso- 
saurus;  8,  duck.      (After  Jordan   and   Kellogg.) 


certainly,  in  the  pectoral  fins  of  fishes.  Com- 
parison does  not  end  here.  We  find  similarities 
in  the  skull  and  back  bones  of  these  same  ani- 
mals; in  the  brain;  in  the  digestive  system;  in 
the  heart  and  blood  vessels;  in  the  muscles. 
Each  of  these  sj^stems  is  very  complex,  but 


THEORY  OF  EVOLUTION  9 

the  same  general  arrangement  is  found  in  all. 
Anyone  familiar  with  the  evidence  will,  I  think, 
probably  reach  the  conclusion  either  that  these 
animals  have  been  created  on  some  precon- 
ceived plan,  or  else  that  they  have  some  other 
bond  that  unites  them;  for  we  find  it  difficult 
to  believe  that  such  complex,  yet  similar  things 
could  have  arisen  independently.  But  we  try 
to  convince  our  students  of  the  truth  of  the 
theory  of  evolution  not  so  much  by  calling  their 
attention  to  this  relation  as  by  tracing  each 
organ  from  a  simple  to  a  complex  structure. 

I  have  never  known  such  a  course  to  fail  in 
its  intention.  In  fact,  I  know  that  the  student 
often  becomes  so  thoroughly  convinced  that 
lie  resents  any  such  attempt  as  that  which  I 
am  about  to  make  to  point  out  that  the  evidence 
for  his  conviction  is  not  above  criticism. 

Because  we  can  often  arrange  the  series  of 
structures  in  a  line  extending  from  the  very 
simple  to  the  more  complex,  we  are  apt  to  be- 
come unduly  impressed  by  this  fact  and  con- 
clude that  if  we  found  the  complete  series  we 
should  find  all  the  intermediate  steps  and  that 
thev  have  arisen  in  the  order  of  their  complex- 


10  THEORY  OF  EVOLUTION 

ity.  This  conclusion  is  not  necessarily  correct. 
Let  me  give  some  examples  that  have  come 
under  my  own  observation.  We  have  bred  for 
five  years  the  wild  fruit  fly  Drosophila  ampelo- 
phila  (fig.  4)   and  we  have  found  over  a  hun- 


Fig.  i.     Drosophila  ampelophila.     a,  Female  and  b,  male. 


dred  and  twenty-five  new  types  that  breed  true. 
Each  has  arisen  independently  and  suddenly. 
Every  part  of  the  body  has  been  affected  by 
one  or  another  of  these  mutations.  For  in- 
stance many  different  kinds  of  changes  have 


THEORY  OF   EVOLUTION 


11 


taken  place  in  the  wings  and  several  of  these 
involve  the  size  of  the  wings.  If  we  arrange 
the  latter  arbitrarily  in  the  order  of  their  size 
there  will  he  an  almost  complete  series  begin- 
ning with  the  normal  wings  and  ending  with 
those  of  apterous  flies.  Several  of  these  types 
are  represented  in  figure  5.  The  order  in  which 
these  mutations  occurred  bears  no  relation  to 


Fio.  5.  Mutants  of  Drosophila  ampelophila  arranged  in 
order  of  size  of  wings:  (a)  cut;  (l>)  beaded;  (c)  stumpy; 
(<1)   another  individual  of  stumpy;   (f)   vestigial   (g)   apterous. 


12  THEORY  OF  EVOLUTION 

their  size;  each  originated  independently  from 
the  wild  type. 

The  wings  of  the  wild  fly  are  straight  (fig. 
4).  Several  types  have  arisen  in  which  the 
wings  are  bent  upwards  and  in  the  most  ex- 
treme type  the  wings  are  curled  over  the  back, 
as  seen  in  figure  54  (g),  yet  there  is  no  histori- 
cal connection  between  these  stages. 

Mutations  have  occurred  involving  the  pig- 
mentation of  the  body  and  wings.  The  head 
and  thorax  of  the  wild  Drosophila  ampelophila 
are  grayish  yellow,  the  abdomen  is  banded  with 
yellow  and  black,  and  the  wings  are  gray. 
There  have  appeared  in  our  cultures  several 
kinds  of  darker  types  ranging  to  almost  black 
flies  (fig.  20)  and  to  lighter  types  that  are 
quite  yellow.  If  put  in  line  a  series  may  be 
made  from  the  darkest  flies  at  one  end  to  the 
light  yellow  flies  at  the  other.  These  types,  with 
the  fluctuations  that  occur  within  each  type, 
furnish  a  complete  series  of  gradations;  yet 
historically  they  have  arisen  independently  of 
each  other. 

Many  changes  in  eye  color  have  appeared. 
As  many  as  thirty  or  more  races  differing  in  eye 


THEORY  OF  EVOLUTION  13 

color  are  now  maintained  in  our  cultures. 
Some  of  them  are  so  similar  that  they  can 
scarcely  he  separated  from  each  other.  It  is 
easily  possible  beginning  with  the  darkest  eye 
color,  sepia,  which  is  deep  brown,  to  pick  out  a 
perfectly  graded  series  ending  with  pure  white 
eyes.  But  such  a  serial  arrangement  would 
give  a  totally  false  idea  of  the  way  the  different 
types  have  arisen;  and  any  conclusion  based 
on  the  existence  of  such  a  series  might  very 
well  be  entirely  erroneous,  for  the  fact  that  such 
a  series  exists  bears  no  relation  to  the  order  in 
which  its  members  have  appeared. 

Suppose  that  evolution  "in  the  open"  had 
taken  place  in  the  same  way,  by  means  of  dis- 
continuous variation.  What  value  then  would 
the  evidence  from  comparative  anatomy  have 
in  so  far  as  it  is  based  on  a  continuous  series  of 
variants  of  any  organ? 

Xo  one  familiar  with  the  entire  evidence  will 
doubt  for  a  moment  that  these  12.5  races  of 
Drosophila  ampelophila  belong  to  the  same 
species  and  have  had  a  common  origin,  for  while 
they  may  differ  mainly  in  one  thing  they  are 
extremely  alike  in  a  hundred  other  things,  and 


11  THEORY  OF  EVOLUTION 

in  the  general  relation  of  the  parts  to  each 
other. 

It  is  in  this  sense  that  the  evidence  from 
comparative  anatomy  can  be  used  I  think  as 
an  argument  for  evolution.  It  is  the  resem- 
blances that  the  animals  or  plants  in  any  group 
have  in  common  that  is  the  basis  for  such  a  con- 
clusion; it  is  not  because  we  can  arrange  in  a 
continuous  series  any  particular  variations.  In 
other  words,  our  inference  concerning  the  com- 
mon descent  of  two  or  more  species  is  based  on 
the  totality  of  such  resemblances  that  still  re- 
main in  large  part  after  each  change  has  taken 
place.  In  this  sense  the  argument  from  com- 
parative anatomy,  while  not  a  demonstration, 
carries  with  it,  I  think,  a  high  degree  of 
probability. 

The  Evidence  from  Embryology 

In  passing  from  the  egg  to  the  adult  the 
individual  goes  through  a  series  of  changes. 
In  the  course  of  this  development  we  see  not 
only  the  beginnings  of  the  organs  that  gradu- 
ally enlarge  and  change  into  those  of  the  adult 
animal,  but  also  see  that  organs  appear  and 


THEORY   OF   EVOLUTION  15 

later  disappear  before  the  adult  stage  is 
reached.  We  find,  moreover,  that  the  young 
sometimes  resemble  in  a  most  striking  way  the 
adult  stage  of  groups  that  we  place  lower  in  the 
scale  of  evolution. 

Many  years  before  Darwin  advanced  his 
theory  of  evolution  through  natural  selection, 
the  resemblance  of  the  young  of  higher  ani- 
mals to  the  adults  of  lower  animals  had  at- 
tracted the  attention  of  zoologists  and  various 
views,  often  very  naive,  had  been  advanced 
to  account  for  the  resemblance.  Among 
these  speculations  there  was  one  practically 
identical  with  that  adopted  by  Darwin  and  the 
post-Darwinians,  namely  that  the  higher  ani- 
mals repeat  in  their  development  the  adult 
stages  of  lower  animals.  Later  this  view  be- 
came one  of  the  cornerstones  of  the  theory  of 
organic  evolution.  It  reached  its  climax  in  the 
writings  of  Haeckel,  and  I  think  I  may  add 
without  exaggeration  that  for  twenty-five  years 
it  furnished  the  chief  inspiration  of  the  school 
of  descriptive  embryology.  Today  it  is  taught 
in  practically  all  textbooks  of  biology.  Haeckel 
called  this  interpretation  the  Biogenetic  Law. 


16  THEORY  OF  EVOLUTION 

It  was  recognized,  of  course,  that  many  em- 
bryonic stages  could  not  possibly  represent 
ancestral  animals.  A  young  fish  with  a  huge 
yolk  sac  attached  (fig.  6)  could  scarcely  ever 
have  led  a  happy,  free  life  as  an  adult  individ- 


Fig.  6.     Young  trout   (Trutta  fario)   six  days  after  hatching. 
(After   Ziegler.) 


ual.  Such  stages  were  interpreted,  however, 
as  embryonic  additions  to  the  original  ancestral 
type.  The  embryo  had  done  something  on  its 
own  account. 

In  some  animals  the  young  have  structures 
that  attach  them  to  the  mother,  as  does  the 
placenta  of  the  mammals.  In  other  cases  the 
young  develop  membranes  about  themselves — 
like  the  amnion  of  the  chick  (fig.  7)  and 
mammal — that  would  have  shut  off  an  adult 
animal  from  all  intercourse  with  the  outside 


THEORY  OF   EVOLUTION 


17 


world.  Hundreds  of  such  embryonic  adapta- 
tions are  known  to  embrvologists.  These  were 
explained  as  adaptations  and  as  falsifications 
of  the  ancestral  records. 

At  the  end  of  the  last  century  Weismann  in- 


Fig.    7.      Diagram    of    chick    showing    relations    of    amnion, 
allantois   and   yolk.      (After   Lillie.) 

jected  a  new  idea  into  our  views  concerning 
the  origin  of  variations.  He  urged  that  varia- 
tions are  germinal,  i.e.  they  first  appear  in  the 
egg  and  the  sperm  as  changes  that  later  bring 
about  modifications  in  the  individual.  The 
idea  has  been  fruitful  and  is  generally  accepted 
by  most  biologists  today.     It  means  that  the 


18  THEORY  OF  EVOLUTION 

offspring  of  a  pair  of  animals  are  not  affected 
by  the  structure  or  the  activities  of  their  par- 
ents, but  the  germ  plasm  is  the  unmodified 
stream  from  which  both  the  parent  and  the 
young  have  arisen.  Hence  their  resemblance. 
Xow,  it  has  been  found  that  a  variation  arising 
in  the  germ  plasm,  no  matter  what  its  cause, 
may  affect  any  stage  in  the  development  of  the 
next  individuals  that  arise  from  it.  There  is 
no  reason  to  suppose  that  such  a  change  pro- 
duces a  new  character  that  always  sticks  it- 
self, as  it  were,  on  to  the  end  of  the  old  series. 
This  idea  of  germinal  variation  therefore  car- 
ried with  it  the  death  of  the  older  conception 
of  evolution  by  superposition. 

In  more  recent  times  another  idea  has  be- 
come current,  mainly  due  to  the  work  of 
Bateson  and  of  de  Vries — the  idea  that  varia- 
tions are  discontinuous.  Such  a  conception 
does  not  fall  easily  into  line  with  the  statement 
of  the  biogenetic  "law";  for  actual  experience 
with  discontinuous  variation  has  taught  us  that 
new  characters  that  arise  do  not  add  themselves 
to  the  end  of  the  line  of  already  existing  char- 
acters but  if  thev  affect  the  adult  characters 


THEORY   OF  EVOLUTION  19 

they  change  them  without,  as  it  were,  passing 
through  and  beyond  them. 

I  venture  to  think  that  these  new  ideas  and 


Fig.  S.  Diagram  of  head  of  chick  A  and  15,  showing  gill 
slits,  and  aortic  arches;  and  head  of  fish  C  showing  aortic 
arches.      (After    Hesse.) 

this  new  evidence  have  played  havoc  with  the 
biogenetic  "law".  Nevertheless,  there  is  an  in- 
terpretation of  the  facts  that  is  entirely  eoin- 


Fig.  9.     Human  embryo  showing  gill  slits  and  aortic  arches. 
(  \lt.r   His;   from   .Marshall.) 


20 


THEORY  OF  EVOLUTION 


patible  with  the  theory  of  evolution.     Let  me 
illustrate  this  by  an  example. 

The  embryos  of  the  chick  (fig.  8)  and  of 
man  (fig.  9)  possess  at  an  early  stage  in  their 
development  gill-slits  on  the  sides  of  the  neck 
like  those  of  fishes.     No  one  familiar  with  the 


Fig.    10.      Young   fish,    dorsal    view,   and   side   view,   showing- 
gill  slits.     (After  Kopsch.) 


relations  of  the  parts  will  for  a  moment  doubt 
that  the  gill  slits  of  these  embryos  and  of  the 
fish  represent  the  same  structures.  When  we 
look  further  into  the  matter  we  find  that  young 
fish  also  possess  gill  slits  (fig.  10  and  11) — even 
in  young  stages  in  their  development.    Is  it  not 


THEORY   OF   EVOLUTION 


21 


then  more  probable  that  the  mammal  and 
bird  possess  this  stage  in  their  development 
simply  because  it  has  never  been  lost?  Is  not 
this  a  more  reasonable  view  than  to  suppose 
that  the  gill  slits  of  the  embryos  of  the  higher 
forms  represent  the  adult  gill  slits  of  the  fish 


Fig.   11.     Side  views  of  head  of  embryo  sharks,  showing  gill 
slits. 


that  in  some  mysterious  way  have  been  pushed 
back  into  the  embryo  of  the  bird? 

I  could  give  many  similar  examples.  All 
can  be  interpreted  as  embryonic  survivals 
rather  than  as  phyletic  contractions.  Not  one 
of  them  calls  for  the  latter  interpretation. 

The  study  of  the  cleavage  pattern  of  the 
segmenting  egg  furnishes  the  most  convincing 
evidence  that  a  different  explanation  from  the 
one  stated  in  the  biogenetic  law  is  the  more 
probable  explanation. 


22 


THEORY  OF  EVOLUTION 


It  has  been  found  that  the  cleavage  pattern 
lias  the  same  general  arrangement  in  the  early 
stages  of  flat  worms,  annelids  and  molluscs 
(fig.  12).     Obviously  these  stages  have  never 


Fig.  12.  Cleavage  stages  of  four  types  of  eggs,  showing  the 
origin  of  the  mesenchyme  cells  (stippled)  and  mesoderm  cells 
(darker);  a,  Planarian;  b,  Annelid  (Podarke)  ;  c,  Mollusc 
(Crepidula),  c,  Mollusc   (Unio). 


been  adult  ancestors,  and  obviously  if  their 
resemblance  has  any  meaning  at  all,  it  is  that 
each  group  has  retained  the  same  general  plan 


THEORY  OF  EVOLUTION  83 

of  cleavage,  possessed  by  their  common 
ancestor. 

Accepting  this  view,  let  us  ask,  does  the  evi- 
dence from  embryology  favor  the  theory  of 
evolution?  I  think  that  it  does  very  strongly. 
The  embryos  of  the  mammal,  bird,  and  lizard 
have  gill  slits  today  because  gill  slits  were  pres- 
ent in  the  embryos  of  their  ancestors.  There  is 
no  other  view  that  explains  so  well  their  pres- 
ence in  the  higher  forms. 

Perhaps  someone  will  say,  Well !  is  not  this 
all  that  we  have  contended  for!  Have  you 
not  reached  the  old  conclusion  in  a  roundabout 
way?  I  think  not.  To  my  mind  there  is  a 
wide  difference  between  the  old  statement  that 
the  higher  animals  living  today  have  the  origi- 
nal adult  stages  telescoped  into  their  embryos, 
and  the  statement  that  the  resemblance  be- 
tween certain  characters  in  the  embryos  of 
higher  animals  and  corresponding  stages  in  the 
embryos  of  lower  animals  is  most  plausibly  ex- 
plained by  the  assumption  that  they  have 
descended  from  the  same  ancestors,  and  that 
their  common  structures  are  embryonic  sur- 
vivals. 


24  THEORY  OF  EVOLUTION 

The  Evidence  from  Paleontology 

The  direct  evidence  furnished  by  fossil  re- 
mains is  by  all  odds  the  strongest  evidence  that 
we  have  in  favor  of  organic  evolution.  Paleon- 
tology holds  the  incomparable  position  of  being 
able  to  point  directly  to  the  evidence  showing 
that  the  animals  and  plants  living  in  past  times 
are  connected  with  those  living  at  the  present 
time,  often  through  an  unbroken  series  of 
stages.  Paleontology  has  triumphed  over  the 
weakness  of  the  evidence,  which  Darwin  ad- 
mitted was  serious,  by  filling  in  many  of  the 
missing  links. 

Paleontology  has  been  criticised  on  the 
ground  that  she  cannot  pretend  to  show  the 
actual  ancestors  of  living  forms  because,  if  in 
the  past  genera  and  species  were  as  abundant 
and  as  diverse  as  we  find  them  at  present,  it  is 
very  improbable  that  the  bones  of  any  individual 
that  happened  to  be  preserved  are  the  bones  of 
just  that  species  that  took  part  in  the  evolution. 
Paleontologists  will  freely  admit  that  in  many 
cases  this  is  probably  true,  but  even  then  the 
evidence  is,  I  think,  still  just  as  valuable  and 


THEORY  OF  EVOLUTION  25 

in  exactly  the  same  sense  as  is  the  evidence  from 
comparative  anatomy.  It  suffices  to  know  that 
there  lived  in  the  past  a  particular  "group"  of 
animals  that  had  many  points  in  common  with 
those  that  preceded  them  and  with  those  that 
came  later.  Whether  these  are  the  actual  an- 
cestors or  not  does  not  so  much  matter,  for  the 
view  that  from  such  a  group  of  species  the  later 
species  have  been  derived  is  far  more  probable 
than  any  other  view  that  has  been  proposed. 

With  this  unrivalled  material  and  splendid 
series  of  gradations,  paleontology  has  con- 
structed many  stages  in  the  past  history  of  the 
globe.  But  paleontologists  have  sometimes 
gone  beyond  this  descriptive  phase  of  the  sub- 
ject and  have  attempted  to  formulate  the 
"causes",  "laws"  and  "principles"  that  have  led 
to  the  development  of  their  series.  It  has  even 
been  claimed  that  paleontologists  are  in  an  in- 
comparably better  position  than  zoologists  to 
discover  such  principles,  because  they  know 
both  the  beginning  and  the  end  of  the  evolu- 
tionary series.  The  retort  is  obvious.  In  his 
sweeping  and  jDoetic  vision  the  paleontologist 
may  fail  completely  to  find  out  the  nature  of 


26  THEORY  OF  EVOLUTION 

the  pigments  that  have  gone  into  the  painting 
of  his  picture,  and  he  may  confuse  a  familiarity 
with  the  different  views  he  has  enjoyed  of  the 
canvas  with  a  knowledge  of  how  the  painting 
is  being  done. 

My  good  friend  the  paleontologist  is  in 
greater  danger  than  he  realizes,  when  he  leaves 
descriptions  and  attempts  explanation.  He 
has  no  way  to  check  up  his  speculations  and  it  is 
notorious  that  the  human  mind  without  con- 
trol has  a  bad  habit  of  wandering. 

When  the  modern  student  of  variation  and 
heredity — the  geneticist — looks  over  the  differ- 
ent "continuous"  series,  from  which  certain 
"laws"  and  "principles"  have  been  deduced,  he 
is  struck  by  two  facts:  that  the  gaps,  in  some 
cases,  are  enormous  as  compared  with  the  single 
changes  with  which  he  is  familiar,  and  (what  is 
more  important)  that  they  involve  numerous 
parts  in  many  ways.  The  geneticist  says  to  the 
paleontologist,  since  you  do  not  know,  and 
from  the  nature  of  your  case  can  never  know, 
whether  5^our  differences  are  due  to  one  change 
or  to  a  thousand,  you  can  not  with  certainty 
tell  us   anything   about  the   hereditary   units 


THEORY  OF  EVOLUTION  27 

which  have  made  the  process  of  evolution  possi- 
ble. And  without  this  knowledge  there  can  be 
no  understanding  of  the  causes  of  evolution. 

THE   FOUR   GREAT    HISTORICAL 
SPECULATIONS 

Looking  backward  over  the  history  of  the 
evolution  theory  we  recognize  that  during  the 
hundred  and  odd  years  that  have  elapsed  since 
Buff  on,  there  have  been  four  main  lines  of 
speculation  concerning  evolution.  We  might 
call  them  the  four  great  cosmogonies  or  the 
four  modern  epics  of  evolution. 

The  Environment 

Geojfroy  St.  Hilaire 

About  the  beginning  of  the  last  century 
Geoffroy  St.  Hilaire,  protege,  and  in  some 
respects  a  disciple  of  Buffon,  was  interested  as 
to  how  living  species  are  related  to  the  animals 
and  plants  that  had  preceded  them.  He  was 
familiar  with  the  kind  of  change  that  takes 
place  in  the  embryo  if  it  is  put  into  new  or 
changed  surroundings,  and  from  this  knowl- 
edge he  concluded  that  as  the  surface  of  the 


28  THEORY  OF  EVOLUTION 

earth  slowly  changed — as  the  carbon  dioxide 
contents  in  the  air  altered — as  land  appeared — 
and  as  marine  animals  left  the  water  to  inhabit 
it,  they  or  their  embryos  responded  to  the  new 
conditions  and  those  that  responded  favorably 
gave  rise  to  new  creations.  As  the  environ- 
ment changed  the  fauna  and  flora  changed — 
change  for  change.  Here  we  have  a  picture  of 
progressive  evolution  that  carries  with  it  an 
idea  of  mechanical  necessity.  If  there  is  any- 
thing mystical  or  even  improbable  in  St.  Hi- 
liare's  argument  it  does  not  appear  on  the  sur- 
face ;  for  he  did  not  assume  that  the  response  to 
the  new  environment  was  always  a  favorable 
one  or,  as  we  say,  an  adaptation.  He  expressly 
stated  that  if  the  response  was  unfavorable  the 
individual  or  the  race  died  out.  He  assumed 
that  sometimes  the  change  might  be  favorable, 
i.e.,  that  certain  species,  entire  groups,  would 
respond  in  a  direction  favorable  to  their  exist- 
ence in  a  new  environment  and  these  would 
come  to  inherit  the  earth.  In  this  sense  he  an- 
ticipated certain  phases  of  the  natural  selection 
theory  of  Darwin,  but  only  in  part;  for  his 
picture  is  not  one  of  strife  within  and  without 


THEORY  OF  EVOLUTION  29 

the  species,  but  rather  the  escape  of  the  species 
from  the  old  into  a  new  world. 

If  then  we  recognize  the  intimate  bond  in 
chemical  constitution  of  living  things  and  of  the 
world  in  which  they  develop,  what  is  there  im- 
probable in  St.  Hilaire's  hypothesis?  Why,  in 
a  word  is  not  more  credit  given  to  St.  Hilaire 
in  modern  evolutionary  thought?  The  reasons 
are  to  be  found,  I  think,  first,  in  that  the  evi- 
dence to  which  he  appealed  was  meagre  and 
inconclusive;  and,  second,  in  that  much  of  his 
special  evidence  does  not  seem  to  us  to  be  ap- 
plicable. For  example  the  monstrous  forms 
that  development  often  assumes  in  a  strange 
environment,  and  with  which  every  embryolo- 
gist  is  only  too  familiar,  rarely  if  ever  furnish 
combinations,  as  he  supposed,  that  are  capable 
of  living.  On  the  contrary,  they  lead  rather  to 
the  final  catastrophe  of  the  organism.  And 
lastly,  St.  Hilaire's  appeal  to  sudden  and  great 
transformations,  such  as  a  crocodile's  egg 
hatching  into  a  bird,  has  exposed  his  view  to  too 
easy  ridicule. 

But  when  all  is  said,  St.  Hilaire's  conception 
of  evolution  contains  elements  that  form  the 


30  THEORY  OF  EVOLUTION 

background  of  our  thinking  to-day,  for  taken 
broadly,  the  interaction  between  the  organism 
and  its  environment  was  a  mechanistic  concep- 
tion of  evolution  even  though  the  details  of  the 
theory  were  inadequate  to  establish  his  con- 
tention. 

In  our  own  time  the  French  metaphysician 
Bergson  in  his  Evolution  Creatrice  has  pro- 
posed in  mystical  form  a  thought  that  has  at 
least  a  superficial  resemblance  to  St.  Hilaire's 
conception.  The  response  of  living  things  is  no 
longer  hit  in  one  species  and  miss  in  another; 
it  is  precise,  exact;  yet  not  mechanical  in  the 
sense  at  least  in  which  we  usually  employ  the 
word  mechanical.  For  Bergson  claims  that 
the  one  chief  feature  of  living  material  is  that 
it  responds  favorably  to  the  situation  in  which 
it  finds  itself;  at  least  so  far  as  lies  within  the 
possible  physical  limitations  of  its  organization. 
Evolution  has  followed  no  preordained  plan; 
it  has  had  no  creator ;  it  has  brought  about  its 
own  creation  by  responding  adaptively  to  each 
situation  as  it  arose. 

But  note :  the  man  of  science  believes  that  the 
organism  responds  today  as  it  does,  because  at 


THEORY  OF  EVOLUTION  31 

present  it  has  a  chemical  and  physical  constitu- 
tion that  gives  this  response.  We  find  a  speci- 
fic chemical  composition  and  generally  a  specific 
physical  structure  already  existing.  We  have 
no  reason  to  suppose  that  such  particular  reac- 
tions would  take  place  until  a  specific  chemical 
configuration  had  heen  acquired.  Where  did 
this  constitution  come  from?  This  is  the  ques- 
tion that  the  scientist  asks  himself.  I  suppose 
Bergson  would  have  to  reply  that  it  came  into 
existence  at  the  moment  that  the  first  specific 
stimulus  was  applied.  But  if  this  is  the  answer 
we  have  passed  at  once  from  the  realm  of  obser- 
vation to  the  realm  of  fancy — to  a  realm  that 
is  foreign  to  our  experience ;  for  such  a  view  as- 
sumes that  chemical  and  physical  reactions  are 
guided  by  the  needs  of  the  organism  when  the 
reactions  take  place  inside  living  beings. 

Use  and  Disuse 

From  Lamarck  to  Wcismann 

The  second  of  the  four  great  historical  ex- 
planations appeals  to  a  change  not  immedi- 
ately connected  with  the  outer  world,  but  to 
one  within  the  organism  itself. 


32  THEORY  OF  EVOLUTION 

Practice  makes  perfect  is  a  familiar  adage. 
Not  only  in  human  affairs  do  we  find  that  a 
part  through  use  becomes  a  better  tool  for 
performing  its  task,  and  through  disuse  de- 
generates; but  in  the  field  of  animal  behavior 
we  find  that  many  of  the  most  essential  types 
of  behavior  have  been  learned  through  repeated 
associations  formed  by  contact  with  the  outside. 

It  was  not  so  long  ago  that  we  were  taught 
that  the  instincts  of  animals  are  the  inherited 
experience  of  their  ancestors — lapsed  intelli- 
gence was  the  current  phrase. 

Lamarck's  name  is  always  associated  with 
the  application  of  the  theory  of  the  inheritance 
of  acquired  characters.  Darwin  fully  en- 
dorsed this  view  and  made  use  of  it  as  an  expla- 
nation in  all  of  his  writings  about  animals. 
Today  the  theory  has  few  followers  amongst 
trained  investigators,  but  it  still  has  a  popular 
vogue  that  is  widespread  and  vociferous. 

To  Weismann  more  than  to  any  other  single 
individual  should  be  ascribed  the  disfavor  into 
which  this  view  has  fallen.  In  a  series  of  bril- 
liant essays  he  laid  bare  the  inadequacy  of  the 
supposed  evidence  on  which  the  inheritance  of 


THEORY  OF  EVOLUTION  33 

acquired  characters  rested.  Your  neighbor's 
cat,  for  instance,  has  a  short  tail,  and  it  is  said 
that  it  had  its  tail  pinched  off  by  a  closing  door. 
In  its  litter  of  kittens  one  or  more  is  found 
without  a  tail.  Your  neighbor  believes  that 
here  is  a  case  of  cause  and  effect.  He  may  even 
have  known  that  the  mother  and  grandmother 
of  the  cat  had  natural  tails.  But  it  has  been 
found  that  short  tail  is  a  dominant  character; 
therefore,  until  we  know  who  was  the  father  of 
the  short-tailed  kittens  the  accident  to  its 
mother  and  the  normal  condition  of  her  mater- 
nal ancestry  is  not  to  the  point. 

Weismann  appealed  to  common  sense.  He 
made  few  experiments  to  disprove  Lamarck's 
hypothesis.  True,  he  cut  off  the  tails  of  some 
mice  for  a  few  generations  but  got  no  tailless 
offspring  and  while  he  gives  no  exact  measure- 
ments with  coefficients  of  error  he  did  not  ob- 
serve that  the  tails  of  the  descendants  had 
shortened  one  whit.  The  combs  of  fighting 
cocks  and  the  tails  of  certain  breeds  of  sheep 
have  been  cropped  for  many  generations  and 
the  practice  continues  today,  because  their  tails 
are  still  long.    While  in  Lamarck's  time  there 


34  THEORY  OF  EVOLUTION 

was  no  evidence  opposed  to  his  ingenious  the- 
ory, based  as  it  was  on  an  appeal  to  the  ac- 
knowledged facts  of  improvement  that  take 
place  in  the  organs  of  an  individual  through 
their  own  functioning  (a  fact  that  is  as  obvious 
and  remarkable  today  as  in  the  time  of  La- 
marck), yet  now  there  is  evidence  as  to 
whether  the  effects  of  use  and  disuse  are  inher- 
ited, and  this  evidence  is  not  in  accord  with 
Lamarck's  doctrine. 

THE  UNFOLDING  PRINCIPLE 

Ndgeli  and  Bateson 

I  have  ventured  to  put  down  as  one  of  the 
four  great  historical  explanations,  under  the 
heading  of  the  unfolding  principle,  a  conception 
that  has  taken  protean  forms.  At  one  extreme 
it  is  little  more  than  a  mystic  sentiment  to  the 
effect  that  evolution  is  the  result  of  an  inner 
driving  force  or  principle  which  goes  under 
many  names  such  as  Bildungstrieb,  nisus  for- 
mativus,  vital  force,  and  orthogenesis.  Evolu- 
tionary thought  is  replete  with  variants  of  this 
idea,  often  naively  expressed,  sometimes  uncon- 
sciously implied.     Evolution  once  meant,   in 


THEORY  OF  EVOLUTION  35 

fact,  an  unfolding  of  what  pre-existed  in  the 
egg,  and  the  term  still  carries  with  it  some- 
thing of  its  original  significance. 

Nageli's  speculation  written  several  years 
after  Darwin's  "Origin  of  Species"  may  be 
taken  as  a  typical  case.  Nageli  thought  that 
there  exists  in  living  material  an  innate  power 
to  grow  and  expand.  He  vehemently  pro- 
tested that  he  meant  only  a  mechanical  prin- 
ciple but  as  he  failed  to  refer  such  a  principle 
to  any  properties  of  matter  known  to  physicists 
and  chemists  his  view  seems  still  a  mysterious 
affirmation,  as  difficult  to  understand  as  the 
facts  themselves  which  it  purports  to  explain. 

Nageli  compared  the  process  of  evolution 
to  the  growth  of  a  tree,  whose  ultimate  twigs 
represent  the  living  world  of  species.  Natural 
selection  plays  only  the  role  of  the  gardener 
who  prunes  the  tree  into  this  or  that  shape  but 
who  has  himself  produced  nothing.  As  an 
imaginative  figure  of  speech  Nageli's  compari- 
son of  the  tree  might  even  today  seem  to  hold 
if  we  substituted  "mutations"  for  "growth", 
but  although  we  know  so  little  about  what 
causes  mutations  there  is  no  reason  for  suppos- 


36  THEORY  OF  EVOLUTION 

ing  them  to  be  due  to  an  inner  impulse,  and 
hence  they  furnish  no  justification  for  such  a 
hypothesis. 

In  his  recent  presidential  address  before  the 
British  Association  Bateson  has  inverted  this 
idea.  I  suspect  that  his  effort  was  intended  as 
little  more  than  a  tour  de  force.  He  claims 
for  it  no  more  than  that  it  is  a  possible  line  of 
speculation.  Perhaps  he  thought  the  time  had 
come  to  give  a  shock  to  our  too  confident  views 
concerning  evolution.  Be  this  as  it  may,  he 
has  invented  a  striking  paradox.  Evolution 
has  taken  place  through  the  steady  loss  of  in- 
hibiting factors.  Living  matter  was  stopped 
down,  so  to  speak,  at  the  beginning  of  the 
world.  As  the  stops  are  lost,  new  things 
emerge.  Living  matter  has  changed  only  in 
that  it  has  become  simpler. 

Natural  Selection 

Darwin 

Of  the  four  great  historical  speculations 
about  evolution,  the  doctrine  of  Natural  Selec- 
tion of  Darwin  and  Wallace  has  met  with  the 
most  widespread  acceptance.     In  the  last  lee- 


THEORY  OF  EVOLUTION  !37 

hire  I  intend  to  examine  this  theory  critically. 
Here  we  are  concerned  only  with  its  broadest 
aspects. 

Darwin  appealed  to  chance  variations  as 
supplying  evolution  with  the  material  on  which 
natural  selection  works.  If  we  accept,  for  the 
moment,  this  statement  as  the  cardinal  doctrine 
of  natural  selection  it  may  appear  that  evolu- 
tion is  due,  (1)  not  to  an  orderly  response  of 
the  organism  to  its  environment,  (2)  not 
in  the  main  to  the  activities  of  the  animal 
through  the  use  or  disuse  of  its  parts,  (3)  not 
to  any  innate  principle  of  living  material  itself, 
and  (4)  above  all  not  to  purpose  either  from 
within  or  from  without.  Darwin  made  quite 
clear  what  he  meant  by  chance.  By  chance  he 
did  not  mean  that  the  variations  were  not 
causal.  On  the  contrary  he  taught  that  in 
Science  we  mean  by  chance  only  that  the  par- 
ticular combination  of  causes  that  bring  about 
a  variation  are  not  known.  They  are  accidents, 
ii  is  true,  but  they  are  causal  accidents. 

In  his  famous  book  on  "Animals  and  Plants 
under  Domestication",  Darwin  dwells  at  great 
length   on   the  nature  of  the  conditions   that 


38  THEORY  OF  EVOLUTION 

bring  about  variations.  If  his  views  seem  to  us 
today  at  times  vague,  at  times  problematical, 
and  often  without  a  secure  basis,  nevertheless 
we  find  in  every  instance,  that  Darwin  was 
searching  for  the  physical  causes  of  variation. 
He  brought,  in  consequence,  conviction  to 
many  minds  that  there  are  abundant  indica- 
tions, even  if  certain  proof  is  lacking,  that  the 
causes  of  variation  are  to  be  found  in  natural 
processes. 

Today  the  belief  that  evolution  takes  place 
by  means  of  natural  processes  is  generally  ac- 
cepted. It  does  not  seem  probable  that  we 
shall  ever  again  have  to  renew  the  old  contest 
between  evolution  and  special  creation. 

But  this  is  not  enough.  We  can  never  re- 
main satisfied  with  a  negative  conclusion  of  this 
kind.  We  must  find  out  what  natural  causes 
bring  about  variations  in  animals  and  plants; 
and  we  must  also  find  out  what  kinds  of  varia- 
tions are  inherited,  and  how  they  are  inherited. 
If  the  circumstantial  evidence  for  organic  evo- 
lution, furnished  by  comparative  anatomy, 
embryology  and  paleontology  is  cogent,  we 
should  be  able  to  observe  evolution  going  on  at 


THEORY  OF  EVOLUTION  39 

the  present  time,  i.e.  we  should  be  able  to 
observe  the  occurrence  of  variations  and  their 
transmission.  This  has  actually  been  done  by 
the  geneticist  in  the  study  of  mutations  and 
Mendelian  heredity,  as  the  succeeding  lectures 
will  show. 


CHAPTER  II 

THE    BEARING    OF    MENDEL'S    DISCOVERY 

ON  THE  ORIGIN  OF  HEREDITARY 

CHARACTERS 

Between  the  years  1857  and  1868  Gregor 
Mendel,  Augustinian  monk,  studied  the  hered- 
ity of  certain  characters  of  the  common  edible 
pea,  in  the  garden  of  the  monastery  at  Briinn. 

In  his  account  of  his  work  written  in  1868, 
he  said: 

"It  requires  indeed  some  courage  to  undertake  a 
labor  of  such  a  far-reaching  extent ;  it  appears,  how- 
ever, to  be  the  only  right  way  by  which  we  can  finally 
reach  the  solution  of  a  question  the  importance  of 
which  cannot  be  over-estimated  in  connection  with 
the  history  of  the  evolution  of  organic  forms." 

He  tells  us  also  why  he  selected  peas  for  his 
work : 

"The  selection  of  the  plant  group  which  shall  serve 
for  experiments  of  this  kind  must  be  made  with  all 
possible  care  if  it  be  desired  to  avoid  from  the  outset 
every  risk  of  questionable  results." 

"The    experimental    plants    must    necessarily 
40 


THEORY  OF  EVOLUTION  41 

1.  Possess  constant  differentiating  characters. 

2.  The  hybrids  of  sucli  plants  must,  during  the 
flowering  period,  be  protected  from  the  influence  of  all 
foreign  pollen,  or  be  easily  capable  of  such  protec- 
tion." 

Why  do  biologists  throughout  the  world  to- 
day agree  that  Mendel's  discovery  is  one  of 
first  rank? 

A  great  deal  might  be  said  in  this  connec- 
tion. What  is  essential  may  be  said  in  a  few 
words.  Biology  had  been,  and  is  still,  largely 
a  descriptive  and  speculative  science.  Mendel 
showed  by  experimental  proof  that  heredity 
could  be  explained  by  a  simple  mechanism. 
His  discovery  has  been  exceedingly  fruitful. 

Science  begins  with  naive,  often  mystic  con- 
ceptions of  its  problems.  It  reaches  its  goal 
whenever  it  can  replace  its  early  guessing  by 
verifiable  hypotheses  and  predictable  results. 
This  is  what  Mendel's  law  did  for  heredity. 

Mendel's  First  Discovery — Segregation 

Let  us  turn  to  the  demonstration  of  his  first 
law — the  law  of  segregation.  The  first  case  I 
choose  is  not  the  one  given  by  Mendel  but  one 
worked  out  later  by  Correns.     If  the  common 


42 


THEORY  OF  EVOLUTION 


garden  plant  called  four  o'clock  (Mirabilis  jal- 
apa)  with  red  flowers  is  crossed  to  one  having 
white  flowers,  the  offspring  are  pink  (fig.  13). 
The  hybrid,  then,  is  intermediate  in  the  color  of 


yp  ^  tfL 


Fig.  13.     Diagram  illustrating  a  cross  between  a  red  (dark) 
and  a  white  variety  of  four  o'clock   (Mirabilis  jalapa). 


its  flowers  between  the  two  parents.  If  these 
hybrids  are  inbred  the  offspring  are  white,  pink 
and  red,  in  the  proportion  of  1:2:1.  All  of 
these  had  the  same  ancestry,  yet  they  are  of 
three  different  kinds.    If  we  did  not  know  their 


THEORY  OF  EVOLUTION  43 

history  it  would  be  quite  impossible  to  state 
what  the  ancestry  of  the  white  or  of  the  red 
had  been,  for  they  might  just  as  well  have  come 
from  pure  white  and  pure  red  ancestors  respec- 
tively as  to  have  emerged  from  the  pink  hy- 
brids. Moreover,  when  we  test  them  we  find 
that  they  are  as  pure  as  are  white  or  red  flower- 
ing plants  that  have  had  all  white  or  all  red 
flowering  ancestors. 

Mendel's  Law  explains  the  results  of  this 
cross  as  shown  in  figure  14. 

The  egg  cell  from  the  white  parent  carries 
the  factor  for  white,  the  pollen  cell  from  the  red 
parent  carries  the  factor  for  red.  The  hybrid 
formed  by  their  union  carries  both  factors. 
The  result  of  their  combined  action  is  to  pro- 
duce flowers  intermediate  in  color. 

When  the  hybrids  mature  and  their  germ 
cells  (eggs  or  pollen)  ripen,  each  carries  only 
one  of  these  factors,  either  the  red  or  the  white, 
but  not  both.  In  other  words,  the  two  factors 
that  have  been  brought  together  in  the  hybrid 
separate  in  its  germ  cells.  Half  of  the  egg 
cells  are  white  bearing,  half  red  bearing.  Half 
of  the  pollen  cells  are  white  bearing,  half  red 


44  THEORY  OF  EVOLUTION 

bearing.  Chance  combinations  at  fertilization 
give  the  three  classes  of  individuals  of  the  sec- 
ond generation. 

The  white  flowering  plants  should  forever 
breed  true,  as  in  fact  they  do.  The  red  flowering 


o 


PARENTS 


<-  o. 

t 

_  o 
o  #__# 


■0      O 


-.^. 


Fig.   14.     Diagram  illustrating  the  history  of  the   factors  in 
the  germ  cells  of  the  cross  shown  in  Fig.  13. 

plants  also  breed  true.  The  pink  flowering 
plants,  having  the  same  composition  as  the  hy- 
brids of  the  first  generation,  should  give  the 
same  kind  of  result.  They  do,  indeed,  give  this 
result  i.e.  one  white  to  two  pink  to  one  red 
flowered  offspring. 


THEORY  OF  EVOLUTION  45 

Another  case  of  the  same  kind  is  known  to 
breeders  of  poultry.  One  of  the  most  beautiful 
of  the  domesticated  breeds  is  known  as  the  An- 


t 


Fig.  15.  Diagram  illustrating  a  cross  between  special  races 
of  white  and  black  fowls,  producing  the  blue  (here  gray) 
Andalusian. 


dalusian.  It  is  a  slate  blue  bird  shading  into 
blue-black  on  the  neck  and  back.  Breeders 
know  that  these  blue  birds  do  not  breed  true 
but  produce  white,  black,  and  blue  offspring. 


46 


THEORY  OF  EVOLUTION 


The  explanation  of  the  failure  to  produce  a 
pure  race  of  Andalusians  is  that  they  are  like 
the  pink  flowers  of  the  four  o'clock,  i.e.,  they  are 
a  hybrid  type  formed  by  the  meeting  of  the 
white  and  the  black  germ  cells.  If  the  whites 
produced  by  the  Andalusians  are  bred  to  the 


Fig.  16.  Diagram  showing  history  of  germ  cells  of  cross  of 
Fig.  15.  The  larger  circles  indicate  the  color  of  the  birds; 
their  enclosed  small  circles  the  nature  of  the  factors  in  the 
germ  cells  of  such  birds. 


blacks  (both  being  pure  strains),  all  the  off- 
spring will  be  blue  (fig.  15)  ;  if  these  blues  are 
inbred  they  will  give  1  white,  to  2  blues,  to  1 


THEORY  OF  EVOLUTION 


47 


black.  In  other  words,  the  factor  for  white  and 
the  factor  for  black  separate  in  the  germ  cells 
of  the  hybrid  Andalusian  birds  (fig.  16) . 

The  third  case  is  Mendel's  classical  case 
of  yellow  and  green  peas  (fig.  17) .  He  crossed 
a  plant  belonging  to  a  race  having  yellow  peas 
with  one  having  green  peas.  The  hybrid  plants 
had  yellow  seeds.  These  hybrids  inbred  gave 
three  yellows  to  one  green.     The  explanation 


*    *    * 


Fig.  17.     Diagram  of  Mendel's  cross  between  yellow   (dom- 
inant) and  green   (recessive)  peas. 


48  THEORY  OF  EVOLUTION 

(fig.  18)  is  the  same  in  principle  as  in  the  pre- 
ceding cases.  The  only  difference  between 
them  is  that  the  hybrid  which  contains  both  the 
yellow  and  the  green  factors  is  in  appearance 


•     0><0     0^ 

Fig.   18.     Diagram  illustrating  the  history  of  the  factors  in 
the  cross   shown  in   Fig.    17. 

not  intermediate,  but  like  the  yellow  parent 
stock.  Yellow  is  said  therefore  to  be  dominant 
and  green  to  be  recessive. 

Another  example  where  one  of  the  con- 
trasted characters  is  dominant  is  shown  by  the 
cross  of  Drosophila  with  vestigial  wings  to 
the  wild  type  with  long  wings  (fig.  19).  The 
Fx  flies  have  long  wings  not  differing  from 
those  of  the  wild  fly,  so  far  as  can  be  observed. 
When  two  such  flies  are  inbred  there  result 
three  long  to  one  vestigial. 


THEORY  OF  EVOLUTION 


49 


dnmnp 
<mnnn> 

Vestigial 


(mmnD 


Gametes  or  F, 


( )  OTTTTTFTD        Eggs 

t 

CTTTTTTTTD      Sperm 


C 


J 


cnnmiD 


c 


amnnD 


CTTTTTTITD 

anmnD 


Fig.  19.  Diagram  illustrating  a  cross  between  a  fly  (Dro- 
sophila  ampelophila)  with  long  wings  and  a  mutant  fly  with 
vestigial  wings. 


50  THEORY  OF  EVOLUTION 

The  question  as  to  whether  a  given  character 
is  dominant  or  recessive  is  a  matter  of  no  theo- 
retical importance  for  the  principle  of  segre- 
gation, although  from  the  notoriety  given  to  it 
one  might  easily  be  misled  into  the  erroneous 
supposition  that  it  was  the  discovery  of  this  re- 
lation that  is  Mendel's  crowning  achievement. 

Let  me  illustrate  by  an  example  in  which  the 
hybrid  standing  between  two  types  overlaps 
them  both.  There  are  two  mutant  races  in  our 
cultures  of  the  fruit  fly  Drosophila  that  have 
dark  body  color,  one  called  sooty,  another  which 
is  even  blacker,  called  ebony  (fig.  20).  Sooty 
crossed  to  ebony  gives  offspring  that  are  inter- 
mediate in  color.  Some  of  them  are  so  much 
like  sooty  that  they  cannot  be  distinguished 
from  sooty.  At  the  other  extreme  some  of  the 
hybrids  are  as  dark  as  the  lightest  of  the  ebony 
flies.  If  these  hybrids  are  inbred  there  is  a  con- 
tinuous series  of  individuals,  sooties,  interme- 
diates and  ebonies.  Which  color  here  shall  we 
call  the  dominant?  If  the  ebony,  then  in  the 
second  generation  we  count  three  ebonies  to 
one  sooty,  putting  the  hybrids  with  the  ebonies. 
If  the  dominant  is  the  sootv  then  we  count  three 


THEORY  OF   EVOLUTION 


51 


sooties  to  one  ebony,  putting  the  hybrids  with 
the  sooties.  The  important  fact  to  find  out  is 
whether  there  actually  exist  three  classes  in  the 

This  can   be   ascertained 


second  generation 


Fig.  20.  Cross  between  two  allelomorphic  races  of  Dro- 
sophila,  sooty  and  ebony,  that  give  a  completely  graded  series 
in  F„. 


52  THEORY  OF  EVOLUTION 

even  when,  as  in  this  case,  there  is  a  perfectly 
graded  series  from  one  end  to  the  other,  by 
testing  out  individually  enough  of  the  flies  to 
show  that  one-fourth  of  them  never  produce 
any  descendants  but  ebonies,  one-fourth  never 
any  but  sooties,  and  one-half  of  them  give  rise 
to  both  ebony  and  sooty. 

Mendel's  Second  Discovery — Independent 
Assortment 

Besides  his  discovery  that  there  are  pairs  of 
characters  that  disjoin,  as  it  were,  in  the  germ 
cells  of  the  hybrid  (law  of  segregation)  Men- 
del made  a  second  discovery  which  also  has 
far-reaching  consequences.  The  following  case 
illustrates  Mendel's  second  law. 

If  a  pea  that  is  yellow  and  round  is  crossed 
to  one  that  is  green  and  wrinkled  (fig.  21),  all 
of  the  offspring  are  yellow  and  round.  Inbred, 
these  give  9  yellow  round,  3  green  round,  3 
yellow  wrinkled,  1  green  wrinkled.  All  the 
yellows  taken  together  are  to  the  green  as  3:  1. 
All  the  round  taken  together  are  to  the  wrin- 
kled as  three  to  one ;  but  some  of  the  yellows  are 
now  wrinkled  and  some  of  the  green  are  now 


THEORY  OF   EVOLUTION 


53 


round.  There  has  been  a  recombination  of  char- 
acters, while  at  the  same  time  the  results,  for 
each  pair  of  characters  taken  separately,  are  in 


PARENTS 


ooo 
ooo 
ooo 


0       © 


Fig.    21.      Cross    between    yellow-round    and    green-wrinkled 
peas,  giving  the  9:  3:  3:  1  ratio  in  F„. 


accord  with  Mendel's  Law  of  Segregation, 
( fig.  22 ) .  The  second  law  of  Mendel  may  be 
called  the  law  of  independent  assortment  of 
different  character  pairs. 

We  can,  as  it  were,  take  the  characters  of 
one  organism  and  recombine  them  with  those 


54< 


THEORY  OF  EVOLUTION 


of  a  different  organism.  We  can  explain  this 
result  as  due  to  the  assortment  of  factors  for 
these  characters  in  the  germ  cells  according  to 
a  definite  law. 

As  a  second  illustration  let  me  take  the  clas- 


O  " 

YR\ 


RENTS 


G  W 


YR  GW 


.        YR 

YR 


YW 
YW 


GR 
GR 


GW 
GW 


YR 
YR 

YR 
YW 

YR 
GR 

YR 
GW 

YW 
YR 

w 

YW 
GR 

O 

GR 
YR 

GR 
YW 

gfTN 

Q 

( 

v^Gwy 

GW 
YR 

Vx*/ 

0% 

VcRy 

o 

Fig.   22.     Diagram   to   show  the  history   of  the    factor   pairs 
yellow-green  and  round-wrinkled  of  the  cross  in  Fig.  21. 


THEORY  OF  EVOLUTION  55 

sic  case  of  the  combs  of  fowls.  If  a  bird  with  a 
rose  comb  is  bred  to  one  with  a  pea  comb  (fig. 
23 ) ,  the  offspring  have  a  comb  different  from 
either.  It  is  called  a  walnut  comb.  If  two 
such  individuals  are  bred  they  give  9  walnut, 


* 


X 


f 


t  f\$ 


Fig.  23.  Cross  between  pea  and  rose  combed  fowls.  (Charts 
of  Baur  and  Goldschmidt.) 

3  rose,  3  pea,  1  single.  This  proportion  shows 
that  the  grandparental  types  differed  in  re- 
spect to  two  pairs  of  characters. 

A  fourth  case  is  shown  in  the  fruit  fly,  where 
an  ebony  fly  with  long  wings  is  mated  to  a  grey 
fly  with  vestigial  wings    (fig.  24).     The  off- 


56 


THEORY  OF  EVOLUTION 


(TTTTTTTTD 

dnmiD) 

Vestigial        Gray 


•  p.    m 


Long  Ebony 


Fig.  24.     Cross  between  long  ebony  and  gray  vestigial  flies. 

spring  are  gray  with  long  wings.  If  these  are 
inbred  they  give  9  gray  long,  3  gray  vestigial, 
3  ebony  long,  1  ebony  vestigial  (figs.  24  and 
25). 


THEORY  OF  EVOLUTION 


57 


The  possibility  of  interchanging  characters 
might  be  illustrated  over  and  over  again.  It  is 
true  not  only  when  two  pairs  of  characters  are 
involved,  but  when  three,  four,  or  more  enter 
the  cross. 


LONG  CRA' 


imam 


ONG         CRAY 


anmnri 

'iiinE" 


Fig.  2r>.     Diagram  to  show  the  history  of  the  factors   in   the 
cross  shown  in  Fig.  24. 


It  is  as  though  we  took  individuals  apart 
and  put  together  |)arts  of  two,  three  or  more 
individuals  by  substituting  one  part  for  another. 


58  THEORY  OF  EVOLUTION 

Not  only  has  this  power  to  make  whatever 
combinations  we  choose  great  practical  impor- 
tance, it  has  even  greater  theoretical  signifi- 
cance; for,  it  follows  that  the  individual  is  not 
in  itself  the  unit  in  heredity,  but  that  within  the 
germ-cells  there  exist  smaller  units  concerned 
with  the  transmission  of  characters. 

The  older  mystical  statement  of  the  individ- 
ual as  a  unit  in  heredity  has  no  longer  any  in- 
terest in  the  light  of  these  discoveries,  except 
as  a  past  phase  of  biological  history.  We  see, 
too,  more  clearly  that  the  sorting  out  of  factors 
in  the  germ  plasm  is  a  very  different  process 
from  the  influence  of  these  factors  on  the  devel- 
opment of  the  organism.  There  is  today  no 
excuse  for  confusing  these  two  problems. 

If  mechanistic  principles  apply  also  to  em- 
bryonic development  then  the  course  of  devel- 
opment is  capable  of  being  stated  as  a  series 
of  chemico-physical  reactions  and  the  "indi- 
vidual" is  merely  a  term  to  exjDress  the  sum 
total  of  such  reactions  and  should  not  be  in- 
terpreted as  something  different  from  or  more 
than  these  reactions.  So  long  as  so  little  is 
known  of  the  actual  processes  involved  in  devel- 


THEORY  OF  EVOLUTION  59 

opment  the  use  of  the  term  "individuality", 
while  giving  the  appearance  of  profundity,  in 
reality  often  serves  merely  to  cover  ignorance 
and  to  make  a  mystery  out  of  a  mechanism. 

The  Characters  of  Wild  Animals  and 
Plants  Follow  the  Same  Laws  of  In- 
heritance as  do  the  Characters  of 
Domesticated  Animals  and  Plants. 

Darwin  based  many  of  his  conclusions  con- 
cerning variation  and  heredity  on  the  evidence 
derived  from  the  garden  and  from  the  stock 
farm.  Here  he  was  handicapped  to  some  ex- 
tent, for  he  had  at  times  to  rely  on  informa- 
tion much  of  which  was  uncritical,  and  some  of 
which  was  worthless. 

Today  we  are  at  least  better  informed  on 
two  important  points;  one  concerning  the 
hinds  of  variations  that  furnish  to  the  cultiva- 
tor the  materials  for  his  selection;  the  other 
concerning  the  modes  of  inheritance  of  these 
variations.  We  know  now  that  new  charac- 
ters are  continually  appearing  in  domesti- 
cated as  well  as  in  wild  animals  and  plants, 
that  these  characters  are  often  sharply  marked 


60  THEORY  OF  EVOLUTION 

off  from  the  original  characters,  and  whether 
the  differences  are  great  or  whether  they  are 
small  they  are  transmitted  alike  according  to 
Mendel's  law. 

Many  of  the  characteristics  of  our  domesti- 
cated animals  and  cultivated  plants  originated 
long  ago,  and  only  here  and  there  have  the 
records  of  their  first  appearance  been  pre- 
served. In  only  a  few  instances  are  these  rec- 
ords clear  and  definite,  while  the  complete 
history  of  any  large  group  of  our  domesticated 
products  is  unknown  to  us. 

Within  the  last  five  or  six  years,  however, 
from  a  common  wild  species  of  fly,  the  fruit 
fly,  Drosophila  ampelophila,  which  we  have 
brought  into  the  laboratory,  have  arisen  over  a 
hundred  and  twenty-five  new  types  whose 
origin  is  completely  known.  Let  me  call  at- 
tention to  a  few  of  the  more  interesting  of 
these  types  and  their  modes  of  inheritance, 
comparing  them  with  wild  types  in  order  to 
show  that  the  kinds  of  inheritance  found  in  do- 
mesticated races  occur  also  in  wild  types.  The 
results  will  show  beyond  dispute  that  the  char- 
acters of  wild  types  are  inherited  in  precisely 


THEORY  OF  EVOLUTION  61 

the  same  way  as  are  the  characters  of  the  mu- 
tant types — a  fact  that  is  not  generally  appre- 
ciated except  by  students  of  genetics,  although 
it  is  of  the  most  far-reaching  significance  for 
the  theory  of  evolution. 

A  mutant  appeared  in  which  the  eye  color 
of  the  female  was  different  from  that  of  the 
male.  The  eye  color  of  the  mutant  female  is 
a  dark  eosin  color,  that  of  the  male  yellowish 
eosin.  From  the  beginning  this  difference  was 
as  marked  as  it  is  to-day.  Breeding  experi- 
ments show  that  eosin  eye  color  differs  from 
the  red  color  of  the  eye  of  the  wild  fly  by  a 
single  mutant  factor.  Here  then  at  a  single 
step  a  type  appeared  that  was  sexually 
dimorphic. 

Zoologists  know  that  sexual  dimorphism  is 
not  uncommon  in  wild  species  of  animals,  and 
Darwin  proposed  the  theory  of  sexual  selec- 
tion to  account  for  the  difference  between  the 
sexes.  He  assumed  that  the  male  preferred 
certain  kinds  of  females  differing  from  himself 
in  a  particular  character,  and  thus  in  time 
through  sexual  selection,  the  sexes  came  to 
differ  from  each  other. 


62  THEORY  OF  EVOLUTION 

In  the  case  of  eosin  eye  color  no  such  process 
as  that  postulated  by  Darwin  to  account  for 
the  differences  between  the  sexes  was  involved ; 


Fig.  26.     Clover  butterfly   (Colias  philodice)   with  two  types 
of  females,  above;  and  one  type  of  male,  below. 

for  the  single  mutation  that  brought  about  the 
change  also  brought  in  the  dimorphism  with  it. 
In  recent  years  zoologists  have  carefully 
studied  several  cases  in  which  two  types  of  fe- 
male are  found  in  the  same  species.  In  the 
common  clover  butterfly,  there  is  a  yellow  and 
a  white  type  of  female,  while  the  male  is  yellow 
(fig.  26).  It  has  been  shown  that  a  single 
factor  difference  determines  whether  the  female 


THEORY  OF  EVOLUTION 


6$ 


is  yellow  or  white.     The  inheritance  is,  accord- 
ing to  Gerould,  strictly  Mendelian. 

In  Papilio  turnus  there  exist,  in  the  southern 
states,  two  kinds  of  females,  one  yellow  like 
the  male,  one  black  (fig.  27).  The  evidence 
here  is  not  so  certain,  but  it  seems  probable  that 


Fig.  -27.     Papilio  turnus  with  two  types  of  females  above  and 
one   type   of   male   below. 


a  single  factor  difference  determines  whether 
the  female  shall  be  yellow  or  black. 

Finally  in  Papilio  polytes   of  Ceylon  and 
India  three  different  types  of  females  appear, 


U  THEORY  OF  EVOLUTION 

(fig.  28  to  right)  only  one  of  which  is  like  the 
male.  Here  the  analysis  of  the  breeding  data 
shows  the  possibility  of  explaining  this  case 


Fig.  28.     Papilio  polytes,  with  three  types  of  female  to  right 
and  one  type  of  male  above  to  left. 

as  due  to  two  pairs  Mendelian  factors  which 
give  in  combination  the  three  types  of  female. 
Taking  these  cases  together,  they  furnish 
a  much  simpler  explanation  than  the  one  pro- 
posed by  Darwin.  They  show  also  that  char- 
acters like  these  shown  by  wild  species  may 
follow  Mendel's  law. 


THEORY  OF  EVOLUTION 


65 


There  has  appeared  in  our  cultures  a  fly  in 
which  the  third  division  of  the  thorax  with  its 
appendages  has  changed  into  a  segment  like 
the  second   (fig.  29).     It  is  smaller  than  the 


Fig.   29.     Mutant    race   of   fruit   fly    with   intercalated   dupli- 
cate mesothorax   on   dorsal   side. 


normal  mesothorax  and  its  wings  are  imper- 
fectly developed,  but  the  bristles  on  the  upper 
surface  may  have  the  typical  arrangement  of 
the  normal  mesothorax.  The  mutant  shows 
how  great  a  change  may  result  from  a  single 
factor  difference. 

A  factor  that  causes  duplication  in  the  legs 


66 


THEORY  OF  EVOLUTION 


has  also  been  found.  Here  the  interesting 
fact  was  discovered  (Hoge)  that  duplication 
takes  jDlace  only  in  the  cold.  At  ordinary  tem- 
peratures the  legs  are  normal. 

In  contrast  to  the  last  case,  where  a  charac- 
ter is  doubled,  is  the  next  one  in  which  the  eyes 
are  lost  ( fig.  30 ) .  This  change  also  took  place 
at  a  single  step.     All  the  flies  of  this  stock 


Fig.  30.     Mutant  race  of  fruit  fly,  called  eyeless;  a,  a'  normal 
eve. 


however,  cannot  be  said  to  be  eyeless,  since 
many  of  them  show  pieces  of  the  eye — indeed 
the  variation  is  so  wide  that  the  eye  may  even 
.apjDear  like  a  normal  eye  unless  carefully  ex- 


THEORY  OF  EVOLUTION 


6' 


amined.  Formerly  we  were  taught  that  eye- 
less animals  arose  in  caves.  This  case  shows 
that  they  may  also  arise  suddenly  in  glass  milk 
bottles,  by  a  change  in  a  single  factor. 

I  may  recall  in  this  connection  that  wingless 
flies  (fig.  5  f)  also  arose  in  our  cultures  by  a 
single  mutation.  We  used  to  be  told  that 
wingless  insects  occurred  on  desert  islands  be- 
cause those  insects  that  had  the  best  developed 
Mings  had  been  blown  out  to  sea.  Whether 
this  is  true  or  not,  I  will  not  pretend  to  say, 
but  at  any  rate  wingless  insects  may  also  arise, 
not  through  a  slow  process  of  elimination,  but 
at  a  single  step. 

The    preceding   examples   have    all   related 


Fig.  31.  Mutant  race  of  fruit  fly  called  bar  to  the  right 
(normal  to  the  left).  The  eye  is  a  narrow  vertical  bar,  the 
outline  of  the  original  eye   is  indicated. 


68  THEORY  OF  EVOLUTION 

to    recessive    characters.      The    next    one    is 
dominant. 

A  single  male  appeared  with  a  narrow  verti- 
cal red  bar  (fig.  31)  instead  of  the  broad  red 
oval  eye.  Bred  to  wild  females  the  new  char- 
acter was  found  to  dominate,  at  least  to  the 
extent  that  the  eyes  of  all  its  offspring  were 
narrower  than  the  normal  eye,  although  not  so 
narrow  as  the  eye  of  the  pure  stock.     Around 


Fig.  32.     Mutant  race  of  fruit  fly,  called  speck.     There  is  a 
minute  black  speck  at  base  of  wing. 

the  bar  there  is  a  wide  border  that  cor- 
responds to  the  region  occupied  by  the  rest  of 
the  eye  of  the  wild  fly.  It  lacks  however  the 
elements  of  the  eye.  It  is  therefore  to  be 
looked  upon  as  a  rudimentary  organ,  which  is, 
so  to  speak,  a  by-product  of  the  dominant 
mutation. 


THEORY  OF  EVOLUTION 


69 


The  preceding  cases  have  all  involved  rather 
great  changes  in  some  one  organ  of  the  body. 
The  following  three  cases  involve  slight 
changes,  and  yet  follow  the  same  laws  of  inheri- 
tance as  do  the  larger  changes. 

At  the  hase  of  the  wings  a  minute  black 
speck  appeared  ( fig.  32 ) .  It  was  found  to 
be  a  Mendelian  character.  In  another  case  the 
spines  on  the  thorax  became  forked  or  kinky 


Fig.  33.  Mutant  race  of  fruit  fly  called  club.  The  wings 
often  remain  unexpanded  and  two  bristles  present  in  wild 
fly    (h)    are   absent   on  side   of  thorax    (c). 


(fig.  52b).  This  stock  breeds  true,  and  the 
character  is  inherited  in  strictly  Mendelian 
fashion. 

In  a  certain  stock  a  number  of  flies  appeared 


70  THEORY  OF  EVOLUTION 

in  which  the  wing  pads  did  not  expand  (fig. 
33).  It  was  found  that  this  peculiarity  is 
shown  in  only  about  twenty  per  cent  of  the  in- 
dividuals supposed  to  inherit  it.  Later  it  was 
found  that  this  stock  lacked  two  bristles  on  the 
sides  of  the  thorax.  By  means  of  this  knowl- 
edge the  heredity  of  the  character  was  easily 
determined.  It  appears  that  while  the  expan- 
sion of  the  wing  pads  fails  to  occur  once  in  five 
times — probably  because  it  is  an  environmental 
effect  peculiar  to  this  stock, — yet  the  minute 
difference  of  the  presence  or  absence  of  the  two 
lateral  bristles  is  a  constant  feature  of  the  flies 
that  carry  this  particular  factor. 

In  the  preceding  cases  I  have  spoken  as 
though  a  factor  influenced  only  one  part  of  the 
body.  It  would  have  been  more  accurate  to 
have  stated  that  the  chief  effect  of  the  factor 
was  observed  in  a  particular  part  of  the  body. 
Most  students  of  genetics  realize  that  a  factor 
difference  usually  affects  more  than  a  single 
character.  For  example,  a  mutant  stock  called 
rudimentary  wings  has  as  its  principle  character- 
istic very  short  wings  (fig.  34) .  But  the  factor 
for  rudimentary  wings  also  produces  other  ef- 


THEORY  OF  EVOLUTION  71 

fects  as  well.  The  females  are  almost  com- 
pletely sterile,  while  the  males  are  fertile.  The 
viability  of  the  stock  is  poor.  When  flies  with 
rudimentary  wings  are  put  into  competition 


Fig.  34.     Mutant  race  of  fruit  fly,  called  rudimentary. 


with  wild  flies  relatively  few  of  the  rudimentary 
flies  come  through,  especially  if  the  culture  is 
crowded.  The  hind  legs  are  also  shortened. 
All  of  these  effects  are  the  results  of  a  single 
factor-difference. 

One  may  venture  the  guess  that  some  of  the 
specific  and  varietal  differences  that  are  char- 


72  THEORY  OF  EVOLUTION 

acteristic  of  wild  types  and  which  at  the  same 
time  appear  to  have  no  survival  value,  are  only 
by-products  of  factors  whose  most  important 
effect  is  on  another  part  of  the  organism  where 
their  influence  is  of  vital  importance. 

It  is  well  known  that  systematists  make  use 
of  characters  that  are  constant  for  groups  of 
species,  but  which  do  not  appear  in  themselves 
to  have  an  adaptive  significance.  If  we  may 
suppose  that  the  constancy  of  such  characters 
may  be  only  an  index  of  the  presence  of  a 
factor  whose  chief  influence  is  in  some  other 
direction  or  directions,  some  physiological  in- 
fluence, for  example,  we  can  give  at  least  a 
reasonable  explanation  of  the  constancy  of 
such  characters. 

I  am  inclined  to  think  that  an  overstatement 
to  the  effect  that  each  factor  may  affect  the  en- 
tire body,  is  less  likely  to  do  harm  than  to  state 
that  each  factor  affects  only  a  particular  char- 
acter. The  reckless  use  of  the  phrase  "unit 
character"  has  done  much  to  mislead  the  unini- 
tiated as  to  the  effects  that  a  single  change  in 
the  germ  plasm  may  produce  on  the  organism. 
Fortunately,  the  expression  "unit  character" 


THEORY  OF  EVOLUTION  73 

is  being  less  used  by  those  students  of  genetics 
who  are  more  careful  in  regard  to  the  implica- 
tions of  their  terminology. 

There  is  a  class  of  cases  of  inheritance,  due  to 
the  XY  chromosomes,  that  is  called  sex  linked 
inheritance.  It  is  shown  both  by  mutant  char- 
acters and  characters  of  wild  species. 

For  instance,  white  eye  color  in  Drosoph- 
ila  shows  sex  linked  inheritance.  If  a  white 
eyed  male  is  mated  to  a  wild  red  eyed  female 
(fig.  35)  all  the  offspring  have  red  eyes.  If 
these  are  inbred,  there  are  three  red  to  one 
white  eyed  offspring,  but  white  eyes  occur  only 
in  the  males.  The  grandfather  has  transmitted 
his  peculiarity  to  half  of  his  grandsons,  but  to 
none  of  his  granddaughters. 

The  reciprocal  cross  (fig.  36)  is  also  inter- 
esting. If  a  white  eyed  female  is  bred  to  a  red 
eyed  male,  all  of  the  daughters  have  red  eyes 
and  all  of  the  sons  have  white  e}res.  We  call 
this  criss-cross  inheritance.  If  these  offspring 
are  inbred,  they  produce  equal  numbers  of 
red  eyed  and  white  eyed  females  and  equal 
numbers  of  red  eyed  and  white  eyed  males.  The 
ratio  is  1:  1:  1:  1,  or  ignoring  sex,  2  reds  to 


74 


THEORY  OF  EVOLUTION 


2  whites,  and  not  the  usual  3:  1  Mendelian 
ratio.  Yet,  as  will  be  shown  later,  the  result 
is  in  entire  accord  with  Mendel's  principle  of 
segregation. 


Fig.  35.    Diagram  showing  a  cross  between  a  white  eyed  male 
and  a  red  eyed  female  of  the  fruit  fly.     Sex  linked  inheritance. 


THEORY  OF  EVOLUTION 


75 


P. 


Fi 


Fig.  36.  Diagram  illustrating  a  cross  between  a  red  eyed 
male  and  white  eyed  female  of  the  fruit  fly  (reciprocal  cross 
(of  that  shown  in  Fig.  3.5). 


It  has  been  shown  by  Sturtevant  that  in  a 
wild  species  of  Drosophila,  viz.,  D.  repleta, 
two  varieties  of  individuals  exist,  in  one  of 
which  the  thorax  has  large  splotches  and  in  the 


76  THEORY  OF  EVOLUTION 

other  type  smaller  splotches  (fig.  37).  The 
factors  that  differentiate  these  varieties  are  sex 
linked. 

Certain  types  of  color  blindness  (fig.  38) 
and  certain  other  abnormal  conditions  in  man 
such  as  haemophilia,  are  transmitted  as  sex 
linked  characters. 


il 


/ 
/ 


V  .    m 


Fig.   37.     Two  types   of  markings   on   thorax   of  Drosophila 
repleta,  both  found  "wild".     They  show  sex  linked  inheritance. 


In  domestic  fowls  sex  linked  inheritance  has 
been  found  as  the  characteristic  method  of 
transmission  for  at  least  as  many  as  six  char- 
acters, but  here  the  relation  of  the  sexes  is  in 
a  sense  reversed.  For  instance,  if  a  black 
Langshan  hen  is  crossed  to  a  barred  Plymouth 
Rock   cock    ( fig.    39 ) ,    the    offspring   are    all 


THEORY  OF  EVOLUTION  77 

X© 

X    0  — 

xxxo 

__  so-1 

^^^^xxxxxo 

Fig.  38,  A.     Diagram   illustrating  inheritance  of  color  blind- 
ness  in   man  •   the  iris  of  the  color-blind   eve   is   here  black. 


m  xx 

x    x« 


30CX 


««^^kx  xx  xi  xo 

Fig.  38,  B.     Reciprocal  of  cross  in    Fig.  38  a. 

barred.  If  these  are  inbred  half  of  the  daugh- 
ters are  blaek  and  half  are  barred  all  of  the 
sons  are  barred.  The  grandmother  has  trans- 
mitted her  color  to  half  of  her  granddaughters 
but  to  none  of  her  grandsons. 


78 


THEORY  OF  EVOLUTION 


In  the  reciprocal  cross  (fig.  40)  black  cock 
by  barred  hen,  the  daughters  are  black  and  the 
sons    barred — criss-cross    inheritance.      These 


Fig.  39.  Sex-linked  inheritance  in  domesticated  birds  shown 
here  in  a  cross  between  barred  Plymouth  Rock  male  and  black 
Langshan  female. 


inbred  give  black  hens  and  black  cocks,  barred 
hens  and  barred  cocks. 

There  is  a  case  comparable  to  this  found  in 
a  wild  species  of  moth,  Abraxas  grossulariata. 
A  wild  variation  of  this  type  is  lighter  in  color 


THEORY  OF  EVOLUTION  79 

and  is  known  as  A.  lacticolor.  When  these  two 
types  are  crossed  they  exhibit  exactly  the  same 
type  of  heredity  as  does  the  black-barred  com- 


Fig.  40.     Reciprocal  of  Fig.  39. 


bination  in  the  domestic  fowl.  As  shown  in 
figure  41,  lacticolor  female  bred  to  grossola- 
riata  male  gives  grossnlariata  sons  and  daugh- 
ters. These  inbred  give  grossnlariata  males 
and  females  and  lacticolor  females.  Recipro- 
cally lacticolor  male  by  grossnlariata  female, 


80 


THEORY  OF  EVOLUTION 


(fig.  42)  gives  lacticolor  daughters  and  gros- 
sulariata  sons  and  these  inbred  give  grossu- 
lariata  males  and  females  and  lacticolor  males 
and  females. 


GR05SULARIATO  9 


Fig.  41.     Sex-linked  inheritance  in  the  wild  moth,  Abraxas 
grossulariata    (darker)    and   A.   lacticolor. 


THEORY  OF  EVOLUTION 


81 


It  has  been  found  that  there  may  be  even 
more  than  two  factors  that  show  Mendelian 
segregation  when  brought  together  in  pairs. 
For  example,  in  the  southern  States  there  are 
several  races  of  the  grouse  locust  (Paratettix) 


LACTICOLOK  9 


Fig.  42.     Reciprocal  of  Fig.  41. 


82  THEORY  OF  EVOLUTION 

that  differ  from  each  other  markedly  in  color 
patterns  (fig.  43).  When  any  two  individuals 
of  these  races  are  crossed  they  give,  as  Nabours 
has  shown,  in  F2  a  Mendelian  ratio  of  1:  2:  1. 
It  is  obvious,  therefore,  that  there  are  here  at 
least  nine  characters,  any  two  of  which  be- 
have as  a  Mendelian  pair.     These  races  have 


Fig.  43.     Four  wild  types  of   Paratettix  in   upper  line  with 
three  hybrids  below. 


THEORY  OF  EVOLUTION 


83. 


arisen  in  nature  and  differ  definitely  and  strik- 
ingly from  each  other,  yet  any  two  differ  by 
only  one  factor  difference. 

Similar  relations  have  been  found  in  a  num- 
ber of  domesticated  races.  In  mice  there  is  a 
quadruple  system  represented  by  the  gray  house 
mouse,  the  white  bellied,  the  yellow  and  the 
black  mouse    (fig.   44).      In   rabbits   there   is 


S: 


) 


Fig.  44.  Diagram  illustrating  four  allelomorphs  in  mice,  viz.. 
gray  bellied  gray  (wild  type)  (above,  to  left)  ;  white  bellied 
gray  (above,  to  right)  ;  yellow  (below,  to  right)  ;  and  black 
(below,  to  left). 


probably  a  triple  system,  that  includes  the  al- 
bino, the  Himalayan,  and  the  black  races.     In 


S4  THEORY  OF  EVOLUTION 

"the  silkworm  moth  there  have  been  described 
four  types  of  larvae,  distinguished  by  different 
color  markings,  that  form  a  system  of  quad- 
ruple  allelomorphs.  In  Drosophila  there  is  a 
quintuple  system  of  factors  in  the  sex  chromo- 
some represented  by  eye  colors,  a  triple  system 
of  body  colors,  and  a  triple  system  of  factors  for 
eye  colors  in  the  third  chromosome. 

Mutation  and  Evolution 

What  bearing  has  the  appearance  of  these 
new  types  of  Drosophila  on  the  theory  of  evo- 
lution may  be  asked.  The  objection  has  been 
raised  in  fact  that  in  the  breeding  work  with 
Drosophila  we  are  dealing  with  artificial  and 
unnatural  conditions.  It  has  been  more  than 
implied  that  results  obtained  from  the  breed- 
ing pen,  the  seed  pan,  the  flower  pot  and  the 
milk  bottle  do  not  apply  to  evolution  in  the 
"open",  nature  "at  large"  or  to  "wild"  types. 
To  be  consistent,  this  same  objection  should  be 
extended  to  the  use  of  the  spectroscope  in  the 
study  of  the  evolution  of  the  stars,  to  the  use 
of  the  test  tube  and  the  balance  by  the  chemist, 
of  the  galvanometer  by  the  physicist.  All  these 


THEORY  OF  EVOLUTION  85 

are  unnatural  instruments  used  to  torture  Na- 
ture's secrets  from  her.  I  venture  to  think  that 
the  real  antithesis  is  not  between  unnatural 
and  natural  treatment  of  Nature,  but  rather 
between  controlled  or  verifiable  data  on  the  one 
hand,  and  unrestrained  generalization  on  the 
other. 

If  a  systematist  were  asked  whether  these 
new  races  of  Drosophila  are  comparable  to 
wild  species,  he  would  not  hesitate  for  a  mo- 
ment. He  would  call  them  all  one  species.  If 
he  were  asked  why,  he  would  say,  I  think, 
"These  races  differ  only  in  one  or  two  striking 
points,  while  in  a  hundred  other  respects  they 
are  identical  even  to  the  minutest  details."  He 
would  add,  that  as  large  a  group  of  wild  spe- 
cies of  flies  would  show  on  the  whole  the  reverse 
relations,  viz.,  they  would  differ  in  nearly  every 
detail  and  be  identical  in  only  a  few  points. 
In  all  this  I  entirely  agree  with  the  systematist, 
for  I  do  not  think  such  a  group  of  types  dif- 
fering by  one  character  each,  is  comparable  to 
most  wild  groups  of  species  because  the  differ- 
ence between  wild  species  is  due  to  a  large  num- 
ber of  such  sinele  differences.     The  characters 


86  THEORY  OF  EVOLUTION 

that  have  been  accumulated  in  wild  species  are 
of  significance  in  the  maintenance  of  the  species, 
or  at  least  we  are  led  to  infer  that  even  though 
the  visible  character  that  we  attend  to  may  not 
itself  be  important,  one  at  least  of  the  other 
effects  of  the  factors  that  represent  these  char- 
acters is  significant.  It  is,  of  course,  hardly  to 
be  expected  that  any  random  change  in  as  com- 
plex a  mechanism  as  an  insect  would  improve 
the  mechanism,  and  as  a  matter  of  fact  it  is 
doubtful  whether  any  of  the  mutant  types  so 
far  discovered  are  better  adapted  to  those  con- 
ditions to  which  a  fly  of  this  structure  and  hab- 
its is  already  adjusted.  But  this  is  beside  the 
mark,  for  modern  genetics  shows  very  posi- 
tively that  adaptive  characters  are  inherited  in 
exactly  the  same  way  as  are  those  that  are  not 
adaptive;  and  I  have  already  pointed  out  that 
we  cannot  study  a  single  mutant  factor  without 
at  the  same  time  studying  one  of  the  factors 
responsible  for  normal  characters,  for  the  two 
together  constitute  the  Mendelian  pair. 

And,  finally,  I  want  to  urge  on  your  atten- 
tion a  question  that  we  are  to  consider  in  more 
detail  in  the  last  lecture.     Evolution  of  wild 


THEORY  OF  EVOLUTION  87 

species  appears  to  have  taken  place  by  modify- 
ing and  improving  bit  by  bit  the  structures 
and  habits  that  the  animal  or  plant  already 
possessed.  We  have  seen  that  there  are  thirty 
mutant  factors  at  least  that  have  an  influence 
on  eye  color,  and  it  is  probable  that  there  are 
at  least  as  many  normal  factors  that  are  in- 
volved in  the  production  of  the  red  eye  of  the 
wild  fly. 

Evolution  from  this  point  of  view  has  con- 
sisted largely  in  introducing  new  factors  that 
influence  characters  already  present  in  the  ani- 
mal or  plant. 

Such  a  view  gives  us  a  somewhat  different 
picture  of  the  process  of  evolution  from  the  old 
idea  of  a  ferocious  struggle  between  the  indi- 
viduals of  a  species  with  the  survival  of  the 
fittest  and  the  annihilation  of  the  less  fit.  Evo- 
lution assumes  a  more  peaceful  aspect.  New 
and  advantageous  characters  survive  by  incor- 
porating themselves  into  the  race,  improving  it 
and  opening  to  it  new  opportunities.  In  other 
words,  the  emphasis  may  be  placed  less  on  the 
competition  between  the  individuals  of  a  spe- 
cies (because  the  destruction  of  the  less  fit  does 


88  THEORY  OF  EVOLUTION 

not  in  itself  lead  to  anything  that  is  new)  than 
on  the  appearance  of  new  characters  and  modi- 
fications of  old  characters  that  become  incor- 
porated in  the  species,  for  on  these  depends  the 
evolution  of  the  race. 


CHAPTER  III 

THE  FACTORIAL  THEORY  OF  HEREDITY  AND 
THE  COMPOSITION  OF  THE  GERM  PLASM 

The  discovery  that  Mendel  made  with  edible 
peas  concerning  heredity  has  been  found  to  ap- 
ply everywhere  throughout  the  plant  and 
animal  kingdoms — to  flowering  plants,  to  in- 
sects, snails,  Crustacea,  fishes,  amphibians, 
birds,  and  mammals  ( including  man ) . 

There  must  be  something  that  these  widely 
separated  groups  of  plants  and  animals  have 
in  common — some  simple  mechanism  per- 
haps— to  give  such  definite  and  orderly  series 
of  results.  There  is,  in  fact,  a  mechanism, 
possessed  alike  by  animals  and  plants,  that  ful- 
fills every  requirement  of  Mendel's  principles. 

The  Cellular  Basis  of  Organic  Evolution 
and  Heredity 

In  order  to  appreciate  the  full  force  of  the 
evidence,  let  me  first  pass  rapidly  in  review  a 

89 


90  THEORY  OF  EVOLUTION 

few  familiar,  historical  facts,  that  preceded  the 
discovery  of  the  mechanism  in  question. 

Throughout  the  greater  part  of  the  last  cen- 
tury, while  students  of  evolution  and  of  hered- 


Fig.  45.  Typical  cell  showing  the  cell  wall,  the  protoplasm 
(with  its  contained  materials)  ;  the  nucleus  with  its  contained 
chromatin    and    nuclear    sap.       (After    Dahlgren.) 


ity  were  engaged  in  what  I  may  call  the  more 
general,  or,  shall  I  say,  the  grosser  aspects  of 
the  subject,  there  existed  another  group  of  stu- 
dents who  were  engaged  in  working  out  the 
minute  structure  of  the  material  basis  of  the 
living  organism.  They  found  that  organs  such 
as  the  brain,  the  heart,  the  liver,  the  lungs,  the 
kidneys,  etc.,  are  not  themselves  the  units  of 
structure,  but  that  all  these  organs  can  be  re- 
duced to  a  simpler  unit  that  repeats  itself  a 


THEORY  OF  EVOLUTION  91 

thousand-fold  in  every  organ.  We  call  this 
unit  a  cell  (fig.  45) . 

The  egg  is  a  cell,  and  the  spermatozoon  is  a 
cell.  The  act  of  fertilization  is  the  union  of  two 
cells  (fig.  47,  upper  figure).  Simple  as  the 
process  of  fertilization  appears  to  us  today,  its 
discovery  swept  aside  a  vast  amount  of  mys- 
tical speculation  concerning  the  role  of  the 
male  and  of  the  female  in  the  act  of  procreation. 

Within  the  cell  a  new  microcosm  was  re- 
vealed. Every  cell  was  found  to  contain  a 
spherical  body  called  the  nucleus  (fig.  46a). 
Within  the  nucleus  is  a  network  of  fibres,  a 
sap  fills  the  interstices  of  the  network.  The  net- 
work resolves  itself  into  a  definite  number  of 
threads  at  each  division  of  the  cell  (fig. 
46  b-e).  These  threads  we  call  chromosomes. 
Each  species  of  animals  and  plants  possesses 
a  characteristic  number  of  these  threads  which 
have  a  definite  size  and  sometimes  a  specific 
shape  and  even  characteristic  granules  at  dif- 
ferent levels.  Beyond  this  point  our  strongest 
microscopes  fail  to  penetrate.  Observation  has 
reached,  for  the  time  being,  its  limit. 

The  story  is  taken  up  at  this  point  by  a  new 


92 


THEORY  OF  EVOLUTION 


?S 

AVVft 

Ffi 

'^'koVm  \mlil 

1211 

Uftt 

1 

Fig.  46.  A  series  of  cells  in  process  of  cell  division.  The 
chromosomes  are  the  black  threads  and  rods.  (After 
Dahlgren.) 


set  of  students  who  have  worked  in  an  entirely 
different  field.  Certain  observations  and  ex- 
periments that  we  have  not  time  to  consider 


THEORY  OF  EVOLUTION  93 

now,  led  a  number  of  biologists  to  conclude  that 
the  chromosomes  are  the  bearers  of  the  heredi- 
tary units.  If  so,  there  should  be  many  such 
units  carried  by  each  chromosome,  for  the  num- 
ber of  chromosomes  is  limited  while  the  number 
of  independently  inherited  characters  is  large. 
In  Drosophila  it  has  been  demonstrated  not  only 
that  there  are  exactly  as  many  groups  of  char- 
acters that  are  inherited  together  as  there  are 
pairs  of  chromosomes,  but  even  that  it  is  possi- 
ble to  locate  one  of  these  groups  in  a  particular 
chromosome  and  to  state  the  relative  position 
there  of  the  factors  for  the  characters.  If  the 
validity  of  this  evidence  is  accepted,  the  study 
of  the  cell  leads  us  finally  in  a  mechanical,  but 
not  in  a  chemical  sense,  to  the  ultimate  units 
about  which  the  whole  process  of  the  transmis- 
sion of  the  hereditary  factors  centers. 

But  before  plunging  into  this  somewhat  tech- 
nical matter  (that  is  difficult  only  because  it  is 
unfamiliar),  certain  facts  which  are  familiar 
for  the  most  part  should  be  recalled,  because 
on  these  turns  the  whole  of  the  subsequent 
story. 

The  thousands  of  cells  that  make  up  the  cell- 


94 


THEORY  OF  EVOLUTION 


state  that  we  call  an  animal  or  plant  come  from 
the  fertilized  egg.  An  hour  or  two  after  fer- 
tilization the  egg  divides  into  two  cells  (fig. 
47).      Then   each  half   divides   again.      Each 


. 


Fig.  47.     An  egg,  and  the  division  of  the  egg — the  so-called 
process  of  cleavage.     (After  Selenka.) 


THEORY  OF  EVOLUTION 


95 


quarter  next  divides.  The  process  continues 
until  a  large  number  of  cells  is  formed  and  out 
of  these  organs  mould  themselves. 

At  every  division  of  the  cell  the  chromosomes 
also  divide.  Half  of  these  have  come  from  the 
mother,  half  from  the  father.     Everv  cell  con- 


Fig.  4-8.  Section  of  the  egg  of  the  heetle,  Calligrapha,  show- 
ing the  pigment  at  one  end  where  the  germ  cells  will  later 
develop  as  shown  in  the  other  two  figures.     (After  Hegner.) 


tains,  therefore,  the  sum  total  of  all  the  chro- 
mosomes, and  if  these  are  the  bearers  of  the 
hereditary   qualities,   every   cell   in   the   bod}\ 


96  THEORY  OF  EVOLUTION 

whatever  its  function,  has  a  common  inheri- 
tance. 

At  an  early  stage  in  the  development  of  the 
animal  certain  cells  are  set  apart  to  form  the 
organs  of  reproduction.  In  some  animals  these 
cells  can  be  identified  early  in  the  cleavage 
(fig.  48). 

The  reproductive  cells  are  at  first  like  all  the 
other  cells  in  the  body  in  that  they  contain  a 
full  complement  of  chromosomes,  half  paternal 
and  half  maternal  in  origin  (fig.  49).  They 
divide  as  do  the  other  cells  of  the  body  for  a 
long  time  (fig.  49,  upper  row).  At  each 
division  each  chromosome  splits  lengthwise  and 
its  halves  migrate  to  opposite  poles  of  the  spin- 
dle (fig.  49  c). 

But  there  comes  a  time  when  a  new  process 
appears  in  the  germ  cells  (fig  49  e-h).  It  is 
essentially  the  same  in  the  egg  and  in  the  sperm 
cells.  The  discovery  of  this  process  we  owe  to 
the  laborious  researches  of  many  workers  in 
many  countries.  The  list  of  their  names  is 
long,  and  I  shall  not  even  attempt  to  repeat  it. 
The  chromosomes  come  together  in  pairs  (fig. 
49  a) .  Each  maternal  chromosome  mates  with 
.a  paternal  chromosome  of  the  same  kind. 


THEORY  OF  EVOLUTION 


97 


Fig.  49.  In  the  upper  row  of  the  diagram  a  typical  process 
of  nuclear  division,  such  as  takes  place  in  the  early  germ  cells 
or  in  the  body  cells.  In  the  lower  row  the  separation  of  the 
chromosomes  that  have  paired.  This  sort  of  separation  takes 
place  at  one  of  the  two  reduction  divisions. 


Then  follow  two  rapid  divisions  (fig.  49  f, 
g  and  .50  and  51 ) .  At  one  of  the  divisions  the 
double  chromosomes  separate  so  that  each  re- 
sulting cell  comes  to  contain  some  maternal  and 


98 


THEORY  OF  EVOLUTION 


Fig.  50.  The  two  maturation  divisions  of  the  sperm  cell. 
Four  sperms  result,  each  with  half  (haploid)  the  full  number 
(diploid)  of  chromosomes. 


some  paternal  chromosomes,  i.e.  one  or  the 
other  member  of  each  pair.  At  the  other  di- 
vision each  chromosome  simply  splits  as  in 
ordinary  cell  division. 

The  upshot  of  the  process  is  that  the  ripe 
eggs  (fig.  51)  and  the  ripe  spermatozoa  (fig. 


THEORY  OF  EVOLUTION  99 


/ 

Fig.  51.  The  two  maturation  divisions  of  the  egg.  The  divi- 
sions are  unequal,  so  that  two  small  polar  bodies  are  formed 
one  of  these  subsequently  divides.  The  three  polar  bodies 
and   the  egg   are   comparable   to   the   four  sperms. 

oO)  come  to  contain  only  half  the  total  num- 
ber of  chromosomes. 

When  the  eggs  are  fertilized  the  whole  num- 
ber of  chromosomes  is  restored  again. 

The  Mechanism  of  Mendelian  Heredity 

Discovered  in  the  Behavior  of 

the  Chromosomes 

If  the  factors  in  heredity  are  carried  in  the 
chromosomes  and  if  the  chromosomes  are  defin- 
ite structures,  we  should  anticipate  that  there 
should  be  as  many  groups  of  characters  as 
there  are  kinds  of  chromosomes.     In  onlv  one 


100  THEORY  OF  EVOLUTION 

case  has  a  sufficient  number  of  characters  been 
studied  to  show  whether  there  is  any  corre- 
spondence between  the  number  of  hereditary 
groups  of  characters  and  the  number  of  chro- 
mosomes. In  the  fruit  fly,  Drosophila  ampelo- 
phila,  we  have  found  about  125  characters  that 
are  inherited  in  a  perfectly  definite  way.  On 
the  opposite  page  is  a  list  of  some  of  them. 

It  will  be  observed  in  this  list  that  the  charac- 
ters are  arranged  in  four  groups,  Groups  I, 
II,  III  and  IV.  Three  of  these  groups  are 
equally  large  or  nearly  so ;  Group  IV  contains 
only  two  characters.  The  characters  are  put  into 
these  groups  because  in  heredity  the  members 
of  each  group  tend  to  be  inherited  together, 
i.e.,  if  two  or  more  enter  the  cross  together  they 
tend  to  remain  together  through  subsequent 
generations.  On  the  other  hand,  any  member 
of  one  group  is  inherited  entirely  independently 
of  any  member  of  the  other  groups ;  in  the  same 
way  as  Mendel's  yellow-green  pair  of  charac- 
ters is  inherited  independently  of  the  round- 
wrinkled  pair. 

If  the  factors  for  these  characters  are  car- 
ried by  the  chromosomes,  then  we  should  ex- 


THEORY  OF  EVOLUTION 


101 


Group   I 

Group   II 

Group  III            Group    I\ 

Abnormal 

Antlered 

Band                     Bent 

Bar 

Apterous 

Beaded                 Eyeless 

Bifid 

Arc 

Cream  III 

Bow- 

Balloon 

Deformed 

Cherry 

Black 

Dwarf 

Chrome 

Blistered 

Ebony 

Cleft 

Comma 

Giant 

Club 

Confluent 

Kidney 

Depressed 

Cream  II 

Low  crossing  over 

Dot 

Curved 

Maroon 

Eosin 

Dachs 

Peach 

Facet 

Extra    vein 

Pink 

Forked 

Fringed 

Rough 

Furrowed 

Jaunty 

Safranin 

Fused 

Limited 

Sepia 

Green 

Little  crossover  Sooty 

Jaunty 

Morula 

Spineless 

Lemon 

Olive 

Spread 

Lethals,  13 

Plexus 

Trident 

Miniature 

Purple 

Truncate  intensifier 

Notch 

Speck 

Whitehead 

Reduplicated 

Strap 

White  ocelli 

Ruby 

Streak 

Rudimentary 

Trefoil 

Sable 

Truncate 

Shifted 

Vestigial 

Short 

Skee 

Spoon 

Spot 

Tan 

Truncate  intensifier 

Vermilion 

White 

Yellow 

102  THEORY  OF  EVOLUTION 

pect  that  those  factors  that  are  carried  by  the 
same  chromosome  would  be  inherited  together, 
provided  the  chromosomes  are  definite  struc- 
tures in  the  cell. 

In  the   chromosome  group   of  Drosophila, 

(fig.  52)  there  are  four  pairs  of  chromosomes, 

three  of  nearly  the  same  size  and  one  much 

smaller.    Not  only  is  there  agreement  between 

FEMtLE  M,LE 

Fig.  52.  Chromosomes  (diploid)  of  D.  ampelophila.  The 
sex  chromosomes  are  XX  in  the  female  and  XY  in  the  male. 
There  are  three  other  pairs  of  chromosomes. 

the  number  of  hereditary  groups  and  the  num- 
ber of  the  chromosomes,  but  even  the  size  rela- 
tions are  the  same,  for  there  are  three  great 
groups  of  characters  and  three  pairs  of  large 
chromosomes,  and  one  small  group  of  charac- 
ters and  one  pair  of  small  chromosomes. 


THEORY  OF  EVOLUTION  103 

The  Four  Great  Linkage  Groups  oe 
Drosophila  ampelophila 

The  following  description  of  the  characters 
of  the  wild  fly  may  be  useful  in  connection  with 
the  account  of  the  modifications  of  these  char- 
acters that  appear  in  the  mutants. 

The  head  and  thorax  of  the  wild  fly  are  gray- 
ish-yellow, the  abdomen  is  banded  with  alter- 
nate stripes  of  yellow  and  black.  In  the  male, 
( fig.  4  to  right ) ,  there  are  three  narrow  bands 
and  a  black  tip.  In  the  female  there  are  five 
black  bands  (fig.  4  to  left) .  The  wings  are  gray 
with  a  surface  texture  of  such  a  kind  that  at  cer- 
tain angles  they  are  iridescent.  The  eyes  are  a 
deep,  solid,  brick-red.  The  minute  hairs  that 
cover  the  body  have  a  very  definite  arrange- 
ment that  is  most  obvious  on  the  head  and 
thorax.  There  is  a  definite  number  of  larger 
hairs  called  bristles  or  chaetae  which  have  a 
characteristic  position  and  are  used  for  diagnos- 
tic purposes  in  classifying  the  species.  On  the 
foreleg  of  the  male  there  is  a  comb-like  organ 
formed  by  a  row  of  bristles;  it  is  absent  in  the 
female.  The  comb  is  a  secondary  sexual  char- 
acter, and  it  is,  so  far  as  known,  functionless. 


104  THEORY  OF  EVOLUTION 

Some  of  the  characters  of  the  mutant  types 
are  shown  in  figures  52,  53,  54,  55.  The  draw- 
ing of  a  single  fly  is  often  used  here  to  illustrate 
more  than  one  character.  This  is  done  to  econ- 
omize space,  but  of  course  there  would  be  no 
difficulty  in  actually  bringing  together  in  the 
same  individual  any  two  or  more  characters  be- 
longing to  the  same  group  (or  to  different 
groups).  Without  colored  figures  it  is  not 
possible  to  show  many  of  the  most  striking  dif- 
ferences of  these  mutant  races;  at  most  dark 
and  light  coloring  can  be  indicated  by  the 
shading  of  the  body,  wings,  or  eyes. 

Group  I 

In  the  six  flies  drawn  in  figure  53  there  are 
shown  five  different  wing  characters.  The 
first  of  these  types  (a)  is  called  cut,  because  the 
ends  of  the  wings  look  as  though  they  had  been 
cut  to  a  point.  The  antennae  are  displaced 
downward  and  appressed  and  their  bristle-like 
aristae  are  crumpled. 

The  second  figure  (b)  represents  a  fly  with  a 
notch  in  the  ends  of  the  wings.  This  charac- 
ter is  dominant,  but  the  same  factor  that  pro- 


THEORY  OF  EVOLUTION 


105 


Fig.    53.      Group    I.       (See    text) 


duces  the  notch  in  the  wings  is  also  a  recessive 
lethal  factor;  because  of  this  latter  effect  of  the 
character  no  males  of  this  race  exist,  and  the 
females  of  the  race  are  never  pure  but  hy- 
brid. Every  female  with  notch  wings  bred 
to  a  wild  male,  will  produce  in  equal  num- 
bers notch  winged  daughters  and  daugh- 
ters with  normal  wings.  There  will  be  half  as 
many  sons  as  daughters.     The  explanation  of 


106  THEORY  OF  EVOLUTION 

this  peculiar  result  is  quite  simple.  Every  notch 
winged  female  has  one  X  chromosome  that 
carries  the  factor  for  notch  and  one  X  chromo- 
some that  is  "normal".  Daughters  receiving 
the  former  chromosomes  are  notched  because 
the  factor  for  notch  is  dominant,  but  they  are 
not  killed  since  the  lethal  effect  of  the  notch 
factor  is  recessive  to  the  normal  allelomorph 
carried  by  the  other  chromosome  that  the 
daughters  get  from  their  father.  This 
normal  factor  is  recessive  for  notch  but  domi- 
nant for  life.  This  same  figure  '(b)  is  used 
here  to  show  three  other  sex  linked  characters. 
The  spines  on  the  thorax  are  twisted  or  kinky, 
which  is  due  to  a  factor  called  "forked".  The 
effect  is  best  seen  on  the  thorax,  but  all  spines 
on  the  body  are  similarly  modified;  even  the 
minute  hairs  are  also  affected.  Ruby  eye  color 
might  be  here  represented — if  the  eyes  in  the 
figure  were  colored.  The  lighter  color  of  the 
body  and  antennae  is  intended  to  indicate  that 
the  character  tan  is  also  present.  The  light 
color  of  the  antennae  is  the  most  certain  way  of 
identifying  tan.  The  tan  flies  are  interesting 
because  they  have  lost  the  positive  heliotropism 


THEORY  OF  EVOLUTION  107 

that  is  so  marked  a  feature  in  the  hehavior  of 
D.  ampelophila.  As  this  peculiarity  of  the  tan 
flies  is  inherited  like  all  the  other  sex  linked 
characters,  it  follows  that  when  a  tan  female  is 
bred  to  a  wild  male  all  the  sons  inherit  the  re- 
cessive tan  color  and  indifference  to  light,  while 
the  daughters  show  the  dominant  sex  linked 
character  of  their  father,  i.e.,  they  are  "gray", 
and  go  to  the  light.  Hence  when  such  a  brood 
is  disturbed  the  females  fly  to  the  light,  but  the 
males  remain  behind. 

One  of  the  first  mutants  that  appeared  in 
D.  ampelophila  was  called  rudimentary  on  ac- 
count of  the  condition  of  the  wings  (c).  The 
same  mutation  has  appeared  independently 
several  times.  In  the  drawing  (c)  the  dark 
body  color  is  intended  to  indicate  "sable"  and 
the  lighter  color  of  the  eyes  is  intended  to  indi- 
cate eosin.  This  eye  color,  which  is  an  allelo- 
morph of  white,  is  also  interesting  because  in 
the  female  the  color  is  deeper  than  in  the  male. 
In  other  cases  of  sex  linked  factors  the  char- 
acter is  the  same  in  the  two  sexes. 

In  the  fourth  figure  (d)  the  third  and  fourth 
longitudinal  veins  of  the  wing  are  fused  into 


108  THEORY  OF  EVOLUTION 

one  vein  from  the  base  of  the  wing  to  the  level 
of  the  first  cross-vein  and  in  addition  converge 
and  meet  near  their  outer  ends.  The  shape  of 
the  eye  is  represented  in  the  figure  as  different 
from  the  normal,  due  to  another  factor  called 
"bar".  This  is  a  dominant  character,  the  hybrid 
condition  being  also  narrow,  but  not  so  narrow 
as  the  pure  type.  Vermilion  e}^e  color  might 
also  be  here  represented — due  to  a  factor 
that  has  appeared  independently  on  several 
occasions. 

In  the  fifth  figure  (e)  the  wings  are  shorter 
and  more  pointed  than  in  the  wild  fly.  This 
character  is  called  miniature.  The  light  color 
of  the  drawing  may  be  taken  to  represent  yel- 
low body  color,  and  the  light  color  of  the  eye 
white  eye  color. 

In  the  last  figure  (f)  the  wings  are  repre- 
sented as  pads,  essentially  in  the  same  condi- 
tion that  they  are  in  when  the  fly  emerges  from 
the  pupa  case.  Not  all  the  flies  of  this  stock  have 
the  wings  in  this  condition ;  some  have  fully  ex- 
panded wings  that  appear  normal  in  all  re- 
spects. Nevertheless,  about  the  same  percen- 
tage of  offspring  show  the  pads  irrespective  of 


THEORY  OF  EVOLUTION  109 

whether  the  parents  had  pads  or  expanded 
wings. 

The  flies  of  this  stock  show,  however,  another 
character,  which  is  a  product  of  the  same  factor, 
and  which  is  constant,  i.e.,  repeated  in  all  in- 
dividuals. The  two  bristles  on  the  sides  of 
the  thorax  are  constantly  absent  in  this  race. 
The  lighter  color  of  the  eye  in  the  figure  may 
be  taken  to  indicate  buff — a  faint  yellowish 
color.  The  factor  for  this  eye  color  is  another 
allelomorph  of  white. 

There  are  many  other  interesting  characters 
that  belong  to  the  first  group,  such  as  abnormal 
abdomen,  short  legs,  duplication  of  the  legs, 
etc.  In  fact,  any  part  of  the  body  may  be  af- 
fected by  a  sex-linked  factor. 

Group  II 

In  the  first  figure  (a)  of  figure  54  that 
contains  members  of  Group  II  the  wings  are 
almost  entirely  absent  or  "vestigial".  This 
condition  arose  at  a  single  step  and  breeds 
true,  although  it  appears  to  be  influenced  to 
some  extent  by  temperature,  also  by  modifiers 
that  sometimes  appear  in  the  stock.     Purple 


110 


THEORY  OF  EVOLUTION 


Fig.  54.     Group    II.      (See   text.) 


eye  color  belongs  in  Group  II;  it  resembles 
the  color  of  the  eye  of  the  wild  fly  but  is  darker 
and  more  translucent. 

In  the  second  figure  (b)  the  wing  is  again 
long  and  narrow  and  sometimes  bent  back  on 
itself,  as  shown  here.  In  several  respects  the 
wing  resembles  strap  (d)  but  seems  to  be  due 


THEORY  OF  EVOLUTION  111 

to  another  factor,  called  antler,  insufficiently 
studied  as  yet. 

In  the  third  figure  (c)  the  wings  turn  up  at 
the  end.  This  is  brought  about  by  the  presence 
of  the  factor  called  jaunty. 

In  the  fourth  figure  the  wings  are  long  and 
narrow  and  several  of  the  veins  are  unrepre- 
sented. This  character,  "strap",  is  very  varia- 
ble and  has  not  yet  been  thoroughly  studied. 
On  the  thorax  there  is  a  deep  black  mark  called 
trefoil.  Even  in  the  wild  fly  there  is  a  three 
pronged  mark  on  the  thorax  present  in  many 
individuals.  Trefoil  is  a  further  development 
and  modification  of  this  mark  and  is  due  to  a 
special  factor. 

In  the  fifth  figure  (e)  the  wings  are  arched. 
The  factor  is  called  arc.  The  dark  color  of  the 
body,  and  especially  of  the  wings,  indicates  the 
factor  for  black. 

The  sixth  figure  (f)  shows  the  wings 
"curved"  downwards.  In  addition  there  is 
present  a  minute  black  speck  at  the  base  of 
each  wing,  due  to  another  factor  called  speck. 

In  the  seventh  figure  (g)  the  wing  is  trun- 
cate.    Its  end  is  obliquely  squared  instead  of 


112  THEORY  OF  EVOLUTION 

rounded;  it  may  be  longer  than  the  body,  or 
shorter  when  other  modifying  factors  are  pres- 
ent. The  mutation  that  produces  this  type  of 
wing  is  of  not  infrequent  occurrence.  It  has 
been  shown  by  Muller  and  Altenburg  that 
there  are  at  least  two  factors  that  modify  this 
character — the  chief  factor  is  present  in  the 
second  chromosome;  alone  it  produces  the 
truncate  wing  in  only  a  certain  percentage  of 
cases,  but  when  the  modifiers  are  also  present 
about  ninety  percent  of  the  individuals  may 
show  the  truncate  condition  of  the  wing.  But 
the  presence  of  these  factors  makes  the  stock 
very  infertile,  so  that  it  is  difficult  to  maintain. 
In  the  eighth  figure  (h)  the  legs  are  short- 
ened owing  to  the  absence  of  a  segment  of  the 
tarsus.  The  stock  is  called  dachs — a  nickname 
given  to  it  because  the  short  legs  suggested  the 
dachshund. 

Group  III 

In  figure  55,  (a),  a  mutant  type  called  bi- 
thorax  is  shown.  The  old  metathorax  is  re- 
placed by  another  mesothorax  thrust  in  between 
the  normal  mesothorax  and  the  abdomen.     It 


THEORY  OF  EVOLUTION 


113 


carries  a  pair  of  wings  that  do  not  completely 
unfold.  On  this  new  mesothorax  the  character- 
istic arrangement  of  the  bristles  is  shown.  Thus 
at  a  single  step  a  typical  region  of  the  body 
has  doubled.  The  character  is  recessive. 
The  size  of  the  adult  fly  of  D.  ampelophila 


Fig.  55.     Group  III.     (See  text.) 

varies  greatly  according  to  the  amount  of 
nourishment  obtained  by  the  larva.  After  the 
fly  emerges  its  size  remains  nearly  constant, 
as  in  many  insects.      Two   races   have,   how- 


114  THEORY  OF  EVOLUTION 

ever,  been  separated  by  Bridges  that  are  dif- 
ferent in  size  as  a  result  of  a  genetic  factor. 
The  first  of  these,  called  dwarf,  is  represented 
by  figure  55,  (b). 

The  race  is  minute,  although  of  course  its 
size  is  variable,  depending  on  food  and  other 
conditions.  The  same  figure  shows  the  pres- 
ence of  another  factor,  "sooty",  that  makes  the 
fly  very  dark.  Maroon  eye  color  might  be 
here  represented,  due  to  still  another  factor. 

In  the  third  figure  (c)  the  other  mutation  in 
size  is  shown.  It  is  called  "giant".  The  flies 
are  twice  the  size  of  wild  flies.  An  eye  color, 
called  peach,  might  here  be  represented.  It  is 
an  allelomorph  of  pink. 

In  the  fourth  figure  (d)  the  mutant  called 
dichaete  is  shown.  It  is  characterized  by  the  ab- 
sence of  two  of  the  bristles  on  the  thorax. 
Other  bristles  may  also  be  absent,  but  not  so 
constantly  as  the  two  just  mentioned.  An- 
other effect  of  the  same  factor  is  the  spread-out 
condition  of  the  wings.  The  very  dark  eye 
color  in  this  figure  may  be  taken  to  indicate 
the  presence  of  another  factor,  "sepia",  which 
causes  the  eyes  to  assume  a  brown  color  that 


THEORY  OF  EVOLUTION  115 

becomes  black  with  age.  Most  of  the  other 
mutations  in  eye  color  that  have  occurred  tend 
to  give  a  lighter  color:  this  one,  which  is  also 
recessive,  makes  the  eye  darker. 

In  the  fifth  figure  (e)  the  color  of  the  dark- 
est fly  is  due  to  a  factor  called  ebony,  which  is 
an  allelomorph  of  sooty. 

In  the  sixth  figure  (f )  the  wings  are  beaded, 
i.e.,  the  margin  is  defective  at  intervals,  giving 
a  beaded-like  outline  to  the  wings.  This  con- 
dition is  very  variable  and  much  affected  by 
other  factors  that  influence  the  shape  of  the 
wings.  The  lighter  eye  color  of  the  drawing 
may  be  taken  to  represent  pink. 

In  the  seventh  figure  (g)  the  wings  are 
curled  up  over  the  back.  This  is  a  recessive 
character. 

Group  IV 

Only  two  mutants  have  been  obtained  that  do 
not  belong  to  any  of  the  preceding  groups; 
these  are  put  together  in  Group  IV.  It  has 
been  shown  that  they  are  linked  to  each  other 
and  the  linkage  is  so  close  that  it  has  thus  far 
been  impossible  to  obtain  the  dominant  recessive. 


116 


THEORY  OF  EVOLUTION 


One  of  these  mutants,  called  "eyeless"  (fig.  56, 
(a,  a1 ) ,  is  variable — the  eyes  are  often  entirely 
absent  or  represented  by  one  or  more  groups  of 
ommatidia.     The  outline  of  the  original  eye, 


Fig.  56.     Group  IV.      (See    text.) 

so  to  speak,  is  strongly  marked  out  and  its 
area  might  be  called  a  rudimentary  organ,  if 
such  a  statement  has  any  meaning  here. 

The  other  figure   (b)   represents  "bent",  so 
called  from  the  shape  of  the  wings.    This  mu- 


THEORY  OF  EVOLUTION  117 

taut  is  likewise  very  variable,  often  indistin- 
guishable from  the  wild  type,  yet  when  well 
developed  strikingly  different  from  any  other 
mutant. 

This  brief  account  of  a  few  of  the  mutant 
races  that  can  be  most  easily  represented  by 
uncolored  figures  will  serve  to  show  how  all 
parts  of  the  body  may  change,  some  of  the 
changes  being  so  slight  that  they  would  be 
overlooked  except  by  an  expert,  others  so  great 
that  in  the  character  affected  the  flies  depart 
far  from  the  original  species. 

It  is  important  to  note  that  mutations  in  the 
first  chromosome  are  not  limited  to  any  part 
of  the  body  nor  do  they  affect  more  frequently  a 
particular  part.  The  same  statement  holds 
equally  for  all  of  the  other  chromosomes.  In 
fact,  since  each  factor  may  affect  visibly  sev- 
eral parts  of  the  body  at  the  same  time  there 
are  no  grounds  for  expecting  any  special  rela- 
tion between  a  given  chromosome  and  special 
regions  of  the  body.  It  can  not  too  insistently 
be  urged  that  when  we  say  a  character  is  the 
product  of  a  particular  factor  we  mean  no 
more  than  that  it  is  the  most  conspicuous  effect 
of  the  factor. 


118  THEORY  OF  EVOLUTION 

If,  then,  as  these  and  other  results  to  be  de- 
scribed point  to  the  chromosomes  as  the  bear- 
ers of  the  Mendelian  factors,  and  if,  as  will  be 
shown  presently,  these  factors  have  a  definite 
location  in  the  chromosomes  it  is  clear  that  the 
location  of  the  factors  in  the  chromosomes  bears 
no  spatial  relation  to  the  location  of  the  parts 
of  the  body  to  each  other. 

Localization  of  Factors  in  the 
Chromosomes 

The  Evidence  from  Sex  Linked  Inheritance 
When  we  follow  the  history  of  pairs  of 
chromosomes  we  find  that  their  distribution  in 
successive  generations  is  paralleled  by  the  in- 
heritance of  Mendelian  characters.  This  is  best 
shown  in  the  sex  chromosomes  (fig.  57).  In 
the  female  there  are  two  of  these  chromosomes 
that  we  call  the  X  chromosomes;  in  the  male 
there  are  also  two  but  one  differs  from  those  of 
the  female  in  its  shape,  and  in  the  fact  that  it 
carries  none  of  the  normal  allelomorphs  of  the 
mutant  factors.  It  is  called  the  Y  chromosome. 
The  course  followed  by  the  sex  chromosomes 
and  that  bv  the  characters  in  the  case  of  sex 


THEORY  OF  EVOLUTION  119 


/    \ 


-Gametes 


88       W 


FEMALE  MALE 

Fig.  57.  Scheme  of  sex  determination  in  Drosophila  type. 
Each  mature  egg  contains  one  X,  each  mature  sperm  contains 
one  X,  or  a  Y  chromosome.  Chance  union  of  any  egg  with 
any  sperm  will  give  either  XX  (female)   or  XY   (male). 


linked  inheritance  are  shown  in  the  next  dia- 
gram of  Drosophila  illustrating  a  cross  between 
a  white  eyed  male  and  a  red  eyed  female. 

The  first  of  these  represents  a  cross  between 
a  white  eyed  male  and  a  red  eyed  female  (fig. 
.58,  top  row).  The  X  chromosome  in  the 
male  is  represented  by  an  open  bar,  the  Y 
chromosome  is  bent.  In  the  female  the  two  X 
chromosomes  are  black.  Each  egg  of  such  a 
female  will  contain  one  "black"  X  after  the 
polar  bodies  have  been  thrown  off.   In  the  male 


120 


THEORY  OF  EVOLUTION 


Fig.  58.  Cross  between  white  eyed  male  of  D.  ampelophila 
and  red  eyed  female.  The  sex  chromosomes  are  indicated  by 
the  rods.  A  black  rod  indicates  that  the  chromosome  carries 
the  factor  for  red;  the  open  chromosome  the  factor  for  white 
eye  color. 


THEORY  OF  EVOLUTION  121 

there  will  be  two  classes  of  sperm — the  female- 
producing-,  carrying  the  (open)  X,  and  the 
male-producing,  carrying  the  Y  chromosome. 
Any  egg  fertilized  by  an  X  bearing  sperm  will 
produce  a  female  that  will  have  red  eyes  be- 
cause the  X  (black)  chromosome  it  gets  from 
the  mother  carries  the  dominant  factor  for  red. 
Any  egg  fertilized  by  a  Y-bearing  sperm  will 
produce  a  male  that  will  also  have  red  eyes  be- 
cause he  gets  his  (black)  X  chromosome  from 
his  mother. 

When,  then,  these  two  Fx  flies  (second  row) 
are  inbred  the  following  combinations  are  ex- 
pected. Each  egg  will  contain  a  black  X  (red 
eye  producing)  or  a  white  X  (white  eye  pro- 
ducing) after  the  polar  bodies  have  been  ex- 
truded. The  male  will  produce  two  kinds  of 
sperms,  of  which  the  female  producing  Avill 
contain  a  black  X  (red  eye  producing) .  Since 
any  effg  may  bv  chance  be  fertilized  bv  any 
sperm  there  will  result  the  four  classes  of  indi- 
viduals shown  on  the  bottom  row  of  the  dia- 
gram. All  the  females  will  have  red  eyes, 
because  irrespective  of  the  two  kinds  of  eggs 
involved  all  the  female-producing  sperm  carry 


122  THEORY  OF  EVOLUTION 


Fig.   59.     Cross  between   red   eyed   male   and   white   eyed    fe- 
male; reciprocal  cross  of  Fig.  58. 


THEORY  OF  EVOLUTION  123 

a  black  X.  Half  of  the  males  have  red  eyes 
because  half  of  the  eggs  have  had  each  a  red- 
producing  X  chromosome.  The  other  half  of 
the  males  have  white  eyes,  because  the  other 
half  of  the  eggs  had  each  a  white-producing  X 
chromosome.  Other  evidence  has  shown  that 
the  Y  chromosome  of  the  male  is  indifferent,  so 
far  as  these  Mendelian  factors  are  concerned. 
The  reciprocal  experiment  is  illustrated  in 
figure  59.  A  white  eyed  female  is  mated  to  a 
red  eyed  male  (top  row) .  All  the  mature  eggs 
of  such  a  female  contain  one  white-producing 
X  chromosome  represented  by  the  open  bar 
in  the  diagram.  The  red  eyed  male  contains  fe- 
male-producing X-bearing  sperm  that  carry 
the  factor  for  red  eye  color,  and  male-produc- 
ing Y  chromosomes.  Any  egg  fertilized  by  an 
X-bearing  sperm  will  become  a  red  eyed  female 
because  the  X  chromosome  that  comes  from  the 
father  carries  the  dominant  factor  for  red  eye 
color.  Any  egg  fertilized  by  a  Y-bearing 
sperm  will  become  a  male  with  white  eyes  be- 
cause the  only  X  chromosome  that  the  male 
contains  comes  from  his  mother  and  is  white 
producing. 


1M  THEORY  OF  EVOLUTION 

When  these  two  Fx  flies  are  inbred  (middle 
row)  the  following  combinations  are  expected. 
Half  the  eggs  will  contain  each  a  white  pro- 
ducing X  chromosome  and  half  red  producing. 
The  female-producing  sperms  will  each  con- 
tain a  white  X  and  the  male-producing  sperms 
will  each  contain  an  indifferent  Y  chromosome. 
Chance  meetings  of  egg  and  sperm  will  give  the 
four  F2  classes  (bottom  row).  These  consist 
of  white  eyed  and  red  eyed  females  and  white 
eyed  and  red  eyed  males.  The  ratio  here  is 
1 : 1  and  not  three  to  one  (3:1)  as  in  other 
Mendelian  cases.  But  Mendel's  law  of  segre- 
gation is  not  transgressed,  as  the  preceding 
analysis  has  shown;  for,  the  chromosomes  have 
followed  strictly  the  course  laid  down  on  Men- 
del's principle  for  the  distribution  of  factors. 
The  peculiar  result  in  this  case  is  due  to  the 
fact  that  the  Fx  male  gets  his  single  factor  for 
eye  color  from  his  mother  only  and  it  is  linked 
to  or  contained  in  a  body  (the  X  chromosome) 
that  is  involved  in  producing  the  females,  while 
the  mate  of  this  body — the  Y  chromosome — is 
indifferent  with  regard  to  these  factors,  yet 
active  as  a  mate  to  X  in  synapsis. 


THEORY  OF  EVOLUTION  125 

In  man  there  are  several  characters  that  show 
exactly  this  same  kind  of  inheritance.  Color 
blindness,  or  at  least  certain  kinds  of  color 
blindness,  appear  to  follow  the  same  scheme.  A 
color  blind  father  transmits  through  his  daugh- 
ters his  peculiarity  to  half  of  his  grandsons, 
but  to  none  of  his  grand-daughters  (fig.  38 A) . 

? 

Diploid  Nuclei 

Gametes 
Fertilization 
Zygotes 

Fig.    60.     Diagram   of   sex   determination    in    type    with    XX 
female  and  XO  male   (after  AVilson). 

The  result  is  the  same  as  in  the  case  of  the  white 
eyed  male  of  Drosophila.  Color  blind  women 
are  rather  unusual,  which  is  exjDected  from  the 
method  of  inheritance  of  this  character,  but  in 
the  few  known  cases  where  such  color  blind 
women  have  married  normal  husbands  the  sons 
have  inherited  the  peculiarity  from  the  mother 
( fig.  38B ) .  Here  again  the  result  is  the  same 
as  for  the  similar  combination  in  Drosophila. 


126  THEORY  OF  EVOLUTION 

In  man  the  sex  formula  appears  to  be  XX 
for  the  female  and  XO  for  the  male  (fig.  60) , 
and  since  the  relation  is  essentially  the  same  as 
that  in  Drosophila  the  chromosome  explanation 


/Seac    determination    in   Man  (Wtnitmen&r.) 


,'»!•'/» 


B 


Fig.  61.  Spermatogenesis  in  man.  There  are  47  chromo- 
somes (diploid)  in  the  male.  After  reduction  half  of  the 
sperm  carry  24  chromosomes  (one  of  which  is  X)  and  half 
carry  23  chromosomes  (no  X). 

is  the  same.  According  to  von  Winiwarter 
there  are  48  chromosomes  in  the  female  and  47 
in  the  male  (fig.  61).  After  the  extrusion  of 
the  polar  bodies  there  are  24  chromosomes  in  the 
egg.    In  the  male  at  one  of  the  two  maturation 


THEORY  OF  EVOLUTION'  127 

divisions  the  X  chromosome  passes  to  one  pole 
undivided  (fig.  61,  C).  In  consequence  there 
are  two  classes  of  sperms  in  man;  female  pro- 
ducing containing  24  chromosomes,  and  male 
producing  containing  23  chromosomes.  If  the 
factor  for  color  blindness  is  carried  by  the  X 
chromosome  its  inheritance  in  man  works  out 
on  the  same  chromosome  scheme  and  in  the 
same  way  as  does  white  eye  color  ( or  any  other 
sex  linked  character)  in  the  fly,  for  the  O 
sperm  in  man  is  equivalent  to  the  Y  sperm  in 
the  fly. 

In  these  cases  we  have  been  dealing  with  a 
single  pair  of  characters.  Let  us  now  take  a 
case  where  two  pairs  of  sex  linked  characters 
enter  the  cross  at  the  same  time,  and  preferably 
a  case  where  the  two  recessives  enter  the  cross 
from  the  same  parent. 

If  a  female  with  white  eyes  and  yellow  wings 
is  crossed  to  a  wild  male  with  red  eyes  and  gray 
wings  (fig.  62),  the  sons  are  yellow  and  have 
white  eyes  and  the  daughters  are  gray  and 
have  red  eyes.  If  two  Fi  flies  are  mated  they 
will  produce  the  following  classes. 


YELLOW  WHITE  J 


Y 
W 

Y 

W 

■si 

R 

? 
W 

Y] 
R 

ft 

1 

w 

u 

s. 

_ 

^ 

—> 

uu 

W 

ff 

s 

R 

[? 

"Yl 
R 

ff 

w 

(? 

_ 

u 

_ 

L 

^ 

L 

„ 

u 

YELLOW  WHITE 


t'ELLOW    RED  GREY  WHITE 


Fig.  62.  Cross  between  a  white  eyed,  yellow  winged  female 
of  D.  ampelophila  and  a  red  eyed,  gray  winged  male.  Two 
pairs  of  sex  linked  characters,  viz.,  white-red  and  yellow-gray 
are  involved.     (See  text.) 


THEORY  OF  EVOLUTION  129 

Yellow  Gray  Yellow  Gray 

White  Red  Red  White 


99.%  1.% 

Xot  only  have  the  two  grandparental  combi- 
nations reappeared,  but  in  addition  two  new 
combinations,  Yiz.,  grey  white  and  yellow  red. 
The  two  original  combinations  far  exceed  in 
numbers  the  new  or  exchange  combinations.  If 
we  follow  the  history  of  the  X  chromosomes  we 
discover  that  the  larger  classes  of  grandchildren 
appear  in  accord  with  the  way  in  which  the  X 
chromosomes  are  transmitted  from  one  genera- 
tion to  the  next. 

The  smaller  classes  of  grandchildren,  the  ex- 
change combinations  or  cross-overs,  as  we  call 
them,  can  be  explained  by  the  assumption 
that  at  some  stage  in  their  history  an  inter- 
change of  parts  has  taken  place  between 
the  chromosomes.  This  is  indicated  in  the 
diagrams. 

The  most  important  fact  brought  out  by  the 
experiment  is  that  the  factors  that  went  in  to- 
gether tend  to  stick  together.  It  makes  no 
difference  in  what  combination  the  members  of 


130  THEORY  OF  EVOLUTION 

the  two  pairs  of  characters  enter,  they  tend  to 
remain  in  that  combination. 

If  one  admits  that  the  sex  chromosomes  carry 
these  factors  for  the  sex-linked  characters — 
and  the  evidence  is  certainly  very  strong  in 
favor  of  this  view — it  follows  necessarily  from 
these  facts  that  at  some  time  in  their  history 
there  has  been  an  interchange  between  the  two 
sex  chromosomes  in  the  female. 

There  are  several  stages  in  the  conjugation 
of  the  chromosomes  at  which  such  an  inter- 
change between  the  members  of  a  pair  might 
occur.  There  is  further  a  small  amount  of 
direct  evidence,  unfortunately  very  meagre  at 
present,  showing  that  an  interchange  does 
actually  occur. 

At  the  ripening  period  of  the  germ  cell  the 
members  of  each  pair  of  chromosomes  come  to- 
gether (fig.  49,  e).  In  several  forms  they 
have  been  described  as  meeting  at  one  end  and 
then  progressively  coming  to  lie  side  by  side  as 
shown  in  fig.  63,  e,  f,  g,  h,  i.  At  the  end  of 
the  process  they  apjDear  to  have  completely 
united  along  their  length  (fig.  63,  j,  k,  1).  It 
is  always  a  maternal  and  a  paternal  chromo- 


THEORY  OF  EVOLUTION 


131 


\rr\ 


V 


S7. 


Fig.  63.     Conjugation  of  chromosomes  (side  to  side  union)  in 
the  spermatogenesis  of  Batracoseps.     (After  Janssens.) 


some  that  meet  in  this  way  and  always  two  of 
the  same  kind.  It  has  been  observed  that  as 
the  members  of  a  pair  come  together  they  oc- 
casionally twist  around  each  other  (fig.  63,  g, 
1,  and  64,  and  65) .  In  consequence  a  part  of  one 


132  THEORY  OF  EVOLUTION 

chromosome  comes  to  be  now  on  one  side  and 
now  on  the  other  side  of  its  mate. 

When  the  chromosomes  separate  at  the  next 
division  of  the  germ  cell  the  part  on  one  side 
passes  to  one  pole,  the  part  on  the  other  to  the 


8S85E8328 


——tcpcco 
ooocPH»«« 


Fig.  64.     Scheme  to  illustrate  a  method  of  crossing  over  of 
the  chromosomes. 

opposite  pole,  ( figs.  64  and  65 ) .  Whenever 
the  chromosomes  do  not  untwist  at  this  time 
there  must  result  an  interchange  of  pieces 
"where  they  were  crossed  over  each  other. 

Janssens  has  found  at  the  time  of  separation 


THEORY  OF  EVOLUTION  133 

evidence  in  favor  of  the  view  that  some  such 
interchange  probably  takes  place. 


We  find  this  same  process  of  interchange  of 
characters  taking  place  in  each  of  the  other 


Fig.  65.     Scheme  to  illustrate  double  crossing  over. 

three  groups  of  Drosophila.     An  example  will 
show  this  for  the  Group  II. 

If  a  black  vestigial  male  is  crossed  to  a  gray 
long- winged  female  ( fig.  66 )  the  offspring  are 
gray  long.  If  an  Fj  female  is  back-crossed  to 
a  black  vestigial  male  the  following  kinds  of 
flies  are  produced: 


134  THEORY  OF  EVOLUTION 

Black  Gray  Black  Gray 

vestigial  long  long  vestigial 


83%  1770 

The  combinations  that  entered  are  more  com- 
mon in  the  F2  generations  than  the  cross-over 
classes,  showing  that  there  is  linkage  of  the  fac- 
tors that  entered  together. 

Another  curious  fact  is  brought  out  if  in- 
stead of  back-crossing  the  F:  female  we  back- 
cross  the  Ft  male  to  a  black  vestigial  female. 
Their  offspring  are  now  of  only  two  kinds, 
black  vestigial  and  gray  long.  This  means 
that  in  the  male  there  is  no  crossing-over  or 
interchange  of  pieces.  This  relation  holds  not 
only  for  the  Group  II  but  for  all  the  other 
groups  as  well. 

Why  interchange  takes  place  in  the  female 
of  Drosophila  and  not  in  the  male  we  do  not 
know  at  present.  We  might  surmise  that  when 
in  the  male  the  members  of  a  pair  come  to- 
gether they  do  not  twist  around  each  other, 
hence  no  crossing-over  results. 

Crossing-over  took  place  between  white  and 
yellow  only  once  in  a  hundred  times.  Other 
characters  show  different  values,  but  the  same 


THEORY  OF  EVOLUTION  135 


Black   re  « tibial 


c~nr) 


Gametes  of  Kale 


Gametes   of   Penal  a 


(  UP  c  b  v  )  CZZD  QEZD  (  EQ  QGD 


CZZD   C  I  s  ) 

BLACl  7SSTX0IAL   gut  LOBQ 


HLACI  VESTIGIAL    GRAY  LOK& 


)  cud 

GRIT  VESTIGIAL 


Fir..  66.  Cross  between  black  vestigial  and  gray  long  flies. 
Two  pairs  of  factors  involved  in  the  second  group.  The  F,  fe- 
male is  back  crossed  (to  right)  to  black  vestigial  male;  and 
the  Fj  male  is  back  crossed  to  black  vestigial  female  (to  left). 
Crossing  over  takes  place  in  the  F,  female  but  not  in  the 
F    male. 


value  under  the  same  conditions  is   obtained 
from  the  same  pair  of  characters. 


)  WHITE.  EOS1N.  CHERRY. 
>  ABNOBMAL. 


)  VERMILION. 


I  MINIATURE. 


>  lull. 
S FUSED. 


«.•  PURPLE. 


-SO.4  CURVED. 


-at.o  PINK.  PEACH. 


■vt.  KIDNEY. 


.  EBONY.  SOOTY. 


Fig.  67.  Map  of  four  chromosomes  of  D.  ampelophila  locat- 
ing those  factors  in  each  group  that  have  been  most  fully 
studied. 


THEORY  OF  EVOLUTION  1537 

If  we  assume  that  the  nearer  together  the 
factors  lie  in  the  chromosome  the  less  likely  is 
a  twist  to  occur  between  them,  and  conversely 
the  farther  apart  they  lie  the  more  likely  is 
a  twist  to  occur  between  them,  we  can  under- 
stand how  the  linkage  is  different  for  different 
pairs  of  factors. 

On  this  basis  we  have  made  out  chromosomal 
maps  for  each  chromosome  (fig.  67) .  The  dia- 
gram indicates  those  loci  that  haATe  been  most 
accurately  placed. 

The  Evidence  from  Interference 

There  is  a  considerable  body  of  information 
that  we  have  obtained  that  corroborates  the  lo- 
cation of  the  factors  in  the  chromosome.  This 
evidence  is  too  technical  to  take  up  in  any  de- 
tail, but  there  is  one  result  that  is  so  important 
that  I  must  attempt  to  explain  it.  If,  as  I 
assume,  crossing  over  is  brought  about  by  twist- 
ing of  the  chromosomes,  and  if  owing  to  the 
material  of  the  chromosomes  there  is  a  most 
frequent  distance  of  internode,  then,  when 
crossing  over  between  nodes  takes  place  at 
tame  level  at  a-b  in  figure  68,  the  region  on 


138  THEORY  OF  EVOLUTION 

each  side  of  that  point,  a  to  A  and  b  to  B, 
should  be  protected,  so  to  speak,  from  further 
crossing  over.  This  in  fact  we  have  found  to 
be  the  case.  No  other  explanation  so  far  pro- 
posed will  account  for  this  extraordinary 
relation. 

What   advantage,   may   be   asked,   is   there 
in    obtaining    numerical    data    of    this    kind? 


Fig.  68.  Scheme  to  indicate  that  when  the  members  of  a 
pair  of  chromosomes  cross  (at  a-b)  the  region  on  each  side  is 
protected   inversely  to  the   distance   from   a-b. 

It  is  this: — whenever  a  new  character  appears 
we  need  only  determine  in  which  of  the  four 
groups  it  lies  and  its  distance  from  two  mem- 
bers within  that  group.  With  this  information 
we  can  predict  with  a  high  degree  of  proba- 
bility what  results  it  will  give  with  any  other 
member  of  any  group.  Thus  we  can  do  on 
paper  what  would  require  many  months  of  la- 
bor by  making  the  actual  experiment.  In  a 
word  we  can  predict  what  will  happen  in  a  situ- 
ation where  prediction  is  impossible  without 
this  numerical  information. 


THEORY  OF  EVOLUTION  139 

The  Evidence  from  Non-Disjunction 

In  the  course  of  the  work  on  Drosophila  ex- 
ceptions appeared  in  one  strain  where  certain 
individuals  did  not  conform  to  the  scheme  of 
sex  linked  inheritance.  For  a  moment  the 
hypothesis  seemed  to  fail,  but  a  careful  exami- 
nation led  to  the  suspicion  that  in  this  strain 
something  had  happened  to  the  sex  chromo- 
somes. It  was  seen  that  if  in  some  way  the  X 
chromosomes  failed  to  disjoin  in  certain  eggs, 
the  exceptions  could  be  explained.  The  analy- 
sis led  to  the  suggestion  that  if  the  Y  chromo- 
some had  got  into  the  female  line  the  results 
would  be  accounted  for,  since  its  presence  there 
would  be  expected  to  cause  this  peculiar  non- 
disjunction of  the  X  chromosomes. 

That  this  was  the  explanation  was  shown 
when  the  material  was  examined.  The  females 
that  gave  these  results  were  found  by  Bridges 
to  have  two  X's  and  a  Y  chromosome. 

The  normal  chromosome  group  of  the  fe- 
male is  shown  in  figure  52  and  the  chromosome 
group  of  one  of  the  exceptional  females  is 
shown  in  figure  69.     In  a  female  of  this  kind 


140  THEORY  OF  EVOLUTION 

there  are  three  sex  chromosomes  X  X  Y 
which  are  homologous  in  the  sense  that  in  nor- 
mal individuals  the  two  present  are  mates 
and  separate  at  the  reduction  division.  If  in 
the  X  X  Y  individual  X  and  X  conjugate  and 
separate  at  reduction  and  the  unmated  Y  is  free 
to  move  to  either  pole  of  the  spindle,  two  kinds 
of  mature  eggs  will  result,  viz.,  X  and  XY. 
If,  on  the  other  hand,  X  and  Y  conjugate  and 


? 


2& 


Fig.    69.      Figure    of    the    chromosome    group    of    an    XXY 
female,  that  gives  non-disjunction. 

separate  at  reduction  and  the  remaining  X  is 
free  to  go  to  either  pole,  four  kinds  of  eggs  will 
result— XY—X— XX— Y.  As  a  total  result 
four  kinds  of  eggs  are  expected:  viz.  many 
XY  and  X  eggs  and  a  few  XX  and  Y  eggs. 

These  four  kinds  of  eggs  may  be  fertilized 
either  by  female-producing  sperms  or  male- 


THEORY  OF  EVOLUTION  141 

producing  sperms,  as  indicated  in  the  diagram 
(fig.  70). 


mctf      mc      ym     xv 


Fig.  70.  Scheme  showing  the  results  of  fertilizing  white 
bearing  eggs  (4  kinds)  resulting  from  non-disjunction.  The 
upper  half  of  the  diagram  gives  the  results  when  these  eggs 
are  fertilized  by  normal  red  bearing,  female  producing  sperm, 
the  lower  half  by  normal,  male  producing  sperm. 

If  such  an  XX Y  female  carried  white  bear- 
ing Xs  (open  X  in  the  figures),  and  the  male 


142  THEORY  OF  EVOLUTION 

carried  a  red  bearing  X  (black  X  in  the  fig- 
ures )  it  will  be  seen  that  there  should  result  an 
exceptional  class  of  sons  that  are  red,  and 
an  exceptional  class  of  daughters  that  are 
white.  Tests  of  these  exceptions  show  that 
they  behave  subsequently  in  heredity  as  their 
composition  requires.  Other  tests  may  also 
be  made  of  the  other  classes  of  offspring. 
Bridges  has  shown  that  they  fulfill  all  the  re- 
quirements predicted.  Thus  a  result  that 
seemed  in  contradiction  with  the  chromosome 
hypothesis  has  turned  out  to  give  a  brilliant 
confirmation  of  that  theory  both  genetically 
and  cytologically. 

How  Many  Genetic  Factors  are  there  in 
the  Germ-plasm  of  a  Single  Individual 

In  passing  I  invite  your  attention  to  a 
speculation  based  on  our  maps  of  the  chromo- 
somes— a  speculation  which  I  must  insist  does 
not  pretend  to  be  more  than  a  guess  but  has  at 
least  the  interest  of  being  the  first  guess  that 
we  have  ever  been  in  position  to  make  as  to 
how  many  factors  go  towards  the  makeup  of 
the  germ  plasm. 


THEORY  OF   EVOLUTION  143 

We  have  found  practically  no  factors  less 
than  .04  of  a  unit  apart.  If  our  map  includes 
the  entire  length  of  the  chromosomes  and  if  we 
assume  factors  are  uniformly  distributed  along 
the  chromosome  at  distances  equal  to  the  short- 
est distance  yet  observed,  viz.  .04,  then  we 
can  calculate  roughly  how  many  hereditary 
factors  there  are  in  Drosophila.  The  calcula- 
tion gives  about  7500  factors.  The  reader 
should  be  cautioned  against  accepting  the 
above  assumptions  as  strictly  true,  for  crossing- 
over  values  are  known  to  differ  according  to 
different  environmental  conditions  (as  shown 
by  Bridges  for  age),  and  to  differ  even  in  dif- 
ferent parts  of  the  chromosome  as  a  result  of 
the  presence  of  specific  genetic  factors  (as 
shown  by  Sturtevant).  Since  all  the  chromo- 
somes except  the  X  chromosomes  are  double 
we  must  double  our  estimate  to  give  the  total 
number  of  factors,  but  the  half  number  is  the 
number  of  the  different  kinds  of  factors  of 
Drosophila. 


144  THEORY  OF  EVOLUTION 

Conclusions 

I  have  passed  in  review  a  long  series  of  re- 
searches as  to  the  nature  of  the  hereditary 
material.  We  have  in  consequence  of  this 
work  arrived  within  sight  of  a  result  that 
seemed  a  few  years  ago  far  beyond  our  reach. 
The  mechanism  of  heredity  has,  I  think,  been 
discovered — discovered  not  by  a  flash  of  intui- 
tion but  as  the  result  of  patient  and  careful 
study  of  the  evidence  itself. 

With  the  discovery  of  this  mechanism  I 
venture  the  opinion  that  the  problem  of  hered- 
ity has  been  solved.  We  know  how  the  factors 
carried  by  the  parents  are  sorted  out  to  the 
germ  cells.  The  explanation  does  not  pretend 
to  state  how  factors  arise  or  how  they  influ- 
ence the  development  of  the  embryo.  But 
these  have  never  been  an  integral  part  of  the 
doctrine  of  heredity.  The  problems  which  they 
present  must  be  worked  out  in  their  own  field. 
So,  I  repeat,  the  mechanism  of  the  chromo- 
somes offers  a  satisfactory  solution  of  the  tra- 
ditional problem  of  heredity. 


CHAPTER  IV 

SELECTION  AND  EVOLUTION 

Darwin's  Theory  of  Natural  Selection  still 
holds  today  first  place  in  every  discussion  of 
evolution,  and  for  this  very  reason  the  theory 
calls  for  careful  scrutiny;  for  it  is  not  difficult 
to  show  that  the  expression  "natural  selection" 
is  to  many  men  a  metaphor  that  carries  many 
meanings,  and  sometimes  different  meanings  to 
different  men.  While  I  heartily  agree  with  my 
fellow  biologists  in  ascribing  to  Darwin  him- 
self, and  to  his  work,  the  first  place  in  biological 
philosophy,  yet  recognition  of  this  claim  should 
not  deter  us  from  a  careful  analysis  of  the  situ- 
ation in  the  light  of  work  that  has  been  done 
since  Darwin's  time. 

The  Theory  of  Natural  Selection 

In  his  great  book  on  the  Origin  of  Species, 
Darwin  tried  to  do  two  things:  first,  to  show 
that  the  evidence  bearing  on  evolution  makes 

14S 


1^6  THEORY  OF  EVOLUTION 

that  explanation  probable.  No  such  great 
body  of  evidence  had  ever  been  brought  to- 
gether before,  and  it  wrought,  as  we  all  know, 
a  revolution  in  our  modes  of  thinking. 

Darwin  also  set  himself  the  task  of  showing 
how  evolution  might  have  taken  place.  He 
pointed  to  the  influence  of  the  environment,  to 
the  effects  of  use  and  disuse,  and  to  natural 
selection.  It  is  to  the  last  theory  that  his  name 
is  especially  attached.  He  appealed  to  a  fact 
familiar  to  everyone,  that  no  two  individuals 
are  identical  and  that  some  of  the  differences 
that  they  show  are  inherited.  He  argued  that 
those  individuals  that  are  best  suited  to  their 
environment  are  the  most  probable  ones  to  sur- 
vive and  to  leave  most  offspring.  In  conse- 
quence their  descendants  should  in  time  replace 
through  competition  the  less  well-adapted 
individuals  of  the  species.  This  is  the  process 
Darwin  called  natural  selection,  and  Spencer 
the  survival  of  the  fittest. 

Stated  in  these  general  terms  there  is  noth- 
ing in  the  theory  to  which  anyone  is  likely  to 
take  exception.  But  let  us  examine  the  argu- 
ment more  critically. 


THEORY   OF   EVOLUTION 


147 


If  we  measure,  or  weigh,  or  classify  any 
character  shown  by  the  individuals  of  a  popu- 
lation, we  find  differences.  We  recognize  that 
some  of  the  differences  are  due  to  the  varied 
experiences  that  the  individuals  have  encoun- 


xWsnzj^W 


Fig.    71.      Series   of  leaves   of    a    tree    arranged    according   to 
size.      (After  de   Vries.) 


tered  in  the  course  of  their  lives,  i.e.  to  their  en- 
vironment, but  we  also  recognize  that  some  of 
the  differences  may  be  due  to  individuals  hav- 
ing different  inheritances — different  germ 
plasms.  Some  familiar  examples  will  help 
to  bring  home  this  relation. 

If  the  leaves  of  a  tree  are  arranged  accord- 
ing to  size  (fig.  71),  we  find  a  continuous  ser- 
ies, but  there  are  more  leaves  of  medium  size 
than  extremes.     If  a  lot  of  beans  be  sorted  out 


148 


THEORY  OF  EVOLUTION 


according  to  their  weights,  and  those  between 
certain  weights  put  into  cylinders,  the  cylin- 
ders, when  arranged  according  to  the  size  of  the 
beans,  will  appear  as  shown  in  figure  72.  An 
imaginary  line  running  over  the  tops  of  the 


Fig.  72.  Beans  put  into  cylindrical  jars  according  to  the 
■sizes  of  the  beans.  The  jars  arranged  according  to  size  of 
contained   beans.      (After   de   Vries.) 

piles  will  give  a  curve  (fig.  73)  that  corresponds 
to  the  curve  of  probability  (fig.  74) . 

If  we  stand  men  in  lines  according  to  their 
height  (fig.  75)  we  get  a  similar  arrangement. 


THEORY  OF  EVOLUTION 


149 


Fig.    73.      A    curve    resulting    from    arrangement    of    beans 
according  to  size.      (After  de  Vries.) 

The  differences  in  size  shown  by  the  individ- 
ual beans  or  by  the  individual  men  are  due  in 
part  to  heredity,  in  part  to  the  environment 


-3d         -2d         +6  0  +6 

Fig.   74.     Curve   of  probability. 


►2d 


♦3d 


150  THEORY  OF  EVOLUTION 

in  which  they  have  developed.  This  is  a  famil- 
iar fact  of  almost  every-day  observation.  It 
is  well  shown  in  the  following  example.  In 
figure  76  the  two  boys  and  the  two  varieties  of 


Fig.     75.      Students     arranged     according    to    size.       (After 
Blakeslee.) 


corn,  which  they  are  holding,  differ  in  height. 
The  pedigrees  of  the  boys  (fig.  77)  make  it 
probable  that  their  height  is  largely  inherited 
and  the  two  races  of  corn  are  known  to  belong- 
to  a  tall  and  a  short  race  respectively.  Here, 
then,  the  chief  effect  or  difference  is  due  to 
heredity.  On  the  other  hand,  if  individuals  of 
the  same  race  develop  in  a  favorable  environ- 
ment the  result  is  different  from  the  develop- 


THEORY  OF  EVOLUTION 


151 


ment  in  an  unfavorable  environment,  as  shown 
in  figure  78.     Here  to  the  right  the  corn  is 


Fig.  76.  A  short  and  a  tall  boy  each  holding  a  stalk  of  corn — 
one  stalk  of  a  race  or  short  corn,  the  other  of  tall  corn.  (After 
Blakeslee.) 


152 


THEORY  OF  EVOLUTION 


crowded  and  in  consequence  dwarfed,  while  to 
the  left  the  same  kind  of  corn  has  had  more 
room  to  develop  and  is  taller. 

Darwin  knew  that  if  selection  of  particular 
kinds  of  individuals  of  a  population  takes  place 
the  next  generation  is  affected.  If  the  taller 
men  of  a  community  are  selected  the  average  of 
their  offspring  will  be  taller  than  the  average  of 


P.        5'5"      5'4"        W/,        5'4!i"        STi  57?  571  5T'   6T     5T0"  5T1V 

D       □  □       Do,-.- 

5T  197-  6^.6"         57" 

Fig.    77.      Pedigree    of    boys    shown    in    Fig.    76.       (After 
Blakeslee.) 

the  former  population.  If  selection  for  tallness 
again  takes  place,  still  taller  men  will  on  the 
average  arise.  If,  amongst  these,  selection 
again  makes  a  choice  the  process  would,  he 
thought,  continue  (fig.  79). 

We  now  recognize  that  this  statement  con- 
tains an  important  truth,  but  we  have  found  that 


THEORY  OF  EVOLUTION 


158 


... 


Fig.   78.     A    race  of   corn   reared   under   different   conditions. 


it  contains  only  a  part  of  the  truth.  Any  one 
who  repeats  for  himself  this  kind  of  selection 
experiment  will  find  that  while  his  average  class 
will  often  change  in  the  direction  of  his  selec- 
tion, the  process  slows  down  as  a  rule  rather 
suddenly  (fig.  80).  He  finds,  moreover,  that 
the  limits  of  variability  are  not  necesarily  trans- 
cended as  the  process  continues  even  although 
the  average  may  for  a  while  be  increased.  More 
tall  men  may  be  produced  by  selection  of  this 
kind,  but  the  tallest  men  are  not  necessarily  any 
taller  than  the  tallest  in  the  original  population. 


154> 


THEORY  OF  EVOLUTION 


Selection,  then,  has  not  produced  anything 
new,  but  only  more  of  certain  kinds  of  indi- 


<         \ 

**/ 

-V 

....  V 

x^- 

^     ^^ 

Fig.  79.  Curves- showing  how  (hypothetieally)  selection  might 
be  supposed  to  bring  about  progress  in  direction  of  selection. 
(After  Goldschmidt.) 

viduals.  Evolution,  however,  means  producing 
more  new  things,  not  more  of  what  already 
exists. 

Darwin  seems  to  have  thought  that  the  range 
of  variation  shown  by  the  offspring  of  a  given 
individual  about  that  type  of  individual  would 
be  as  wide  as  the  range  shown  by  the  original 
population  (fig.  79),  but  Galton's  work  has 
made  it  clear  that  this  is  not  the  case  in  a  gen- 
eral or  mixed  population.  If  the  offspring  of 
individuals  continued  to  show,  as  Darwin  seems 
to  have  thought,  as  wide  a  range  on  each  side  of 
their  parents'  size,  so  to  speak,  as  did  the  orig- 
inal population,  then  it  would  follow  that  se- 


THEORY  OF  EVOLUTION 


155 


lection  could  slide  successive  generations  along 
in  the  direction  of  selection. 

Darwin  himself  was  extraordinarily  careful, 
however,  in  the  statements  he  made  in  this  con- 
nection and  it  is  rather  by  implication  than  by 
actual  reference  that  one  can  ascribe  this  mean- 


Fig.  80.  Diagram  illustrating  the  results  of  selection  for 
extra  bristles  in  D.  ampelophila.  Selection  at  first  produces 
decided  effects  which  soon  slow  down  and  then  cease. 
(MacDowell.) 


156  THEORY  OF  EVOLUTION 

ing  to  his  views.  His  contemporaries  and 
many  of  his  followers,  however,  appear  to  have 
accepted  this  sliding  scale  interpretation  as 
the  cardinal  doctrine  of  evolution.  If  this  is 
doubted  or  my  statement  is  challenged  then 
one  must  explain  why  de  Vries'  mutation  the- 
ory met  with  so  little  enthusiasm  amongst  the 
older  group  of  zoologists  and  botanists;  and 
one  must  explain  why  Johannsen's  splendid 
work  met  with  such  bitter  opposition  from 
the  English  school — the  biometricians — who 
amongst  the  post-Darwinian  school  are  as- 
sumed to  be  the  lineal  descendants  of  Darwin. 
And  in  this  connection  we  should  not  forget 
that  just  this  sort  of  process  was  supposed  to 
take  place  in  the  inheritance  of  use  and  disuse. 
What  is  gained  in  one  generation  forms  the 
basis  for  further  gains  in  the  next  generation. 
Now,  Darwin  not  only  believed  that  acquired 
characters  are  inherited  but  turned  more  and 
more  to  this  explanation  in  his  later  writings. 
Let  us,  however,  not  make  too  much  of  the 
matter ;  for  it  is  much  less  important  to  find  out 
whether  Darwin's  ideas  were  vague,  than  it  is 
to  make  sure  that  our  own  ideas  are  clear. 


THEORY  OF  EVOLUTION  157 

If  I  have  made  several  statements  here  that 
appear  dogmatic  let  me  now  attempt  to  justify 
them,  or  at  least  give  the  evidence  which  seems 
to  me  to  make  them  probable. 

The  work  of  the  Danish  botanist,  Johannsen, 
has  given  us  the  most  carefully  analyzed  case 
of  selection  that  has  ever  been  obtained.  There 
are,  moreover,  special  reasons  why  the  mater- 
ial that  he  used  is  better  suited  to  give  definite 
information  than  any  other  so  far  studied. 
Johannsen  worked  with  the  common  bean, 
weighing  the  seeds  or  else  measuring  them. 
These  beans  if  taken  from  many  plants  at  ran- 
dom give  the  typical  curve  of  probability  (fig. 
74) .  The  plant  multiplies  by  self-fertilization. 
Taking  advantage  of  this  fact  Johannsen  kept 
the  seeds  of  each  plant  separate  from  the 
others,  and  raised  from  them  a  new  generation. 
When  curves  were  made  from  these  new  groups 
it  was  found  that  some  of  them  had  different 
modes  from  that  of  the  original  general  popula- 
tion (fig.  81  A-E,  bottom  group).  They 
are  shown  in  the  upper  groups  (A,  B,  C,  D, 
E ) .  But  do  not  understand  me  to  say  that  the 
offspring  of  each  bean  gave  a  different  mode. 


158 


THEORY  OF  EVOLUTION 


I 

Fig.  81.  Pure  lines  of  beans.  The  lower  figure  gives  the 
general  population,  the  other  figures  give  the  pure  lines  within 
the  population.     (After  Johannsen.) 


THEORY  OF  EVOLUTION  159 

On  the  contrary,  some  of  the  lines  would  he 
the  same. 

The  result  means  that  the  general  population 
is  made  up  of  definite  kinds  of  individuals  that 
may  have  been  sorted  out. 

That  his  conclusion  is  correct  is  shown  by 
rearing  a  new  generation  from  any  plant  or  in- 
deed from  several  plants  of  any  one  of  these 
lines.  Each  line  repeats  the  same  modal  class. 
There  is  no  further  breaking  up  into  groups. 
Within  the  line  it  does  not  matter  at  all  whether 
one  chooses  a  big  bean  or  a  little  one — they  will 
give  the  same  result.  In  a  word,  the  germ 
plasm  in  each  of  these  lines  is  pure,  or  homo- 
zygous, as  we  say.  The  differences  that  we  find 
between  the  weights  (or  sizes)  of  the  individual 
beans  are  due  to  external  conditions  to  which 
they  have  been  subjected. 

In  a  word,  Johannsen's  work  shows  that  the 
frequency  distribution  of  a  pure  line  is  due  to 
factors  that  are  extrinsic  to  the  germ  plasm. 
It  does  not  matter  then  which  individuals  in  a 
pure  line  are  used  to  breed  from,  for  they  all 
carry  the  same  germ  plasm. 

We  can  now  understand  more  clearlv  how 


160  THEORY  OF  EVOLUTION 

selection  acting  on  a  general  population  brings 
about  results  in  the  direction  of  selection. 

An  individual  is  picked  out  from  the  popu- 
lation in  order  to  get  a  particular  kind  of  germ 
plasm.  Although  the  different  classes  of  indi- 
viduals may  overlap,  so  that  one  can  not  always 
judge  an  individual  from  its  appearance,  never- 
theless on  the  whole  chance  favors  the  picking 
out  of  the  kind  of  germ  plasm  sought. 

In  species  with  separate  sexes  there  is  the 
further  difficulty  that  two  individuals  must  be 
chosen  for  each  mating,  and  superficial  examin- 
ation of  them  does  not  insure  that  they  belong 
to  the  same  group — their  germ  plasm  cannot 
be  inspected.  Hence  selection  of  biparental 
forms  is  a  precarious  process,  now  going  for- 
ward, now  backwards,  now  standing  still.  In 
time,  however,  the  process  forward  is  almost 
certain  to  take  place  if  the  selection  is  from 
a  heterogeneous  population.  Johannsen's 
work  was  simplified  because  he  started  with 
pure  lines.  In  fact,  had  he  not  done  so  his 
work  would  not  have  been  essentially  different 
from  that  of  any  selection  experiment  of  a  pure 
race  of  animals  or  plants.    Whether  Johannsen 


THEORY  OF  EVOLUTION  161 

realized  the  importance  of  the  condition  or 
not  is  uncertain — curiously  he  laid  no  em- 
phasis on  it  in  the  first  edition  of  his  "Elemente 
der  exakten  Erblichkeitslehre". 

It  has  since  been  pointed  out  by  Jennings 
and  by  Pearl  that  a  race  that  reproduces  by 
self-fertilization  as  does  this  bean,  automati- 
cally becomes  pure  in  all  of  the  factors  that 
make  up  its  germ  plasm.  Since  self-fertiliza- 
tion is  the  normal  process  in  this  bean  the  pur- 
ity of  the  germ  plasm  already  existed  when 
Johannsen  began  to  experiment. 

How     Has     Selection     ix     Domesticated 

Animals  and  Plants  Brought  About 

Its  Results? 

If  then  selection  does  not  bring  about  trans- 
gressive  variation  in  a  general  population,  how 
can  selection  produce  anything  new?  If  it 
can  not  produce  anything  new,  is  there  any 
other  way  in  which  selection  becomes  an  agent 
in  evolution? 

We  can  get  some  light  on  this  question  if  we 
turn  to  what  man  has  done  with  his  domesti- 
cated animals  and  plants.     Through  selection, 


162  THEORY  OF  EVOLUTION 

i.e.,  artificial  selection,  man  has  undoubtedly 
brought  about  changes  as  remarkable  as  any 
shown  by  wild  animals  and  plants.  We  know, 
moreover,  a  good  deal  about  how  these  changes 
have  been  wrought. 

( 1 )  By  crossing  different  wild  species  or  by 
crossing  wild  with  races  already  domesticated 
new  combinations  have  been  made.  Parts  of 
one  individual  have  been  combined  with  parts 
of  others,  creating  new  combinations.  It  is 
possible  even  that  characters  that  are  entirely 
new  may  be  produced  by  the  interaction  of  fac- 
tors brought  into  recombination. 

(2)  New  characters  appear  from  time  to 
time  in  domesticated  and  in  wild  species. 
These,  like  the  mutants  in  Drosophila,  are  fully 
equipped  at  the  start.  Since  they  breed  true 
and  follow  Mendel's  laws  it  is  possible  to  com- 
bine them  with  characters  of  the  wild  type  or 
with  those  of  other  mutant  races. 

Amongst  the  new  mutant  factors  there 
may  be  some  whose  chief  effect  is  on  the  char- 
acter that  the  breeder  is  already  selecting. 
Such  a  modification  will  be  likely  to  attract  at- 
tention.    Superficially  it  may  appear  that  the 


THEORY  OF  EVOLUTION  163 

factor  for  the  original  character  has  varied, 
while  the  truth  may  be  that  another  factor  has 
appeared  that  has  modified  a  character  already 
present.  In  fact,  many  or  all  Mendelian  fac- 
tors that  affect  the  same  organ  may  be  said 
to  be  modifiers  of  each  other's  effects.  Thus 
the  factor  for  vermilion  causes  the  eye  to  be 
one  color,  and  the  factor  for  eosin  another 
color,  while  eosin  vermilion  is  different  from 
both.  Eosin  may  be  said  to  be  a  modifier  of 
vermilion  or  vermilion  of  eosin.  In  general, 
however,  it  is  convenient  to  use  the  term  "modi- 
fier" for  cases  in  which  the  factor  causes  a 
detectable  change  in  a  character  already  pres- 
ent or  conspicuous. 

One  of  the  most  interesting,  and  at  the  same 
time  most  treacherous,  kinds  of  modifying 
factors  is  that  which  produces  an  effect  only 
when  some  other  factor  is  present.  Thus 
Bridges  has  shown  that  there  is  a  factor  called 
"cream"  that  does  not  affect  the  red  color  of 
the  eye  of  the  wild  fly,  yet  makes  "eosin"  much 
paler  (fig.  82).  Another  factor  "whiting" 
which  produces  no  effect  on  red  makes  eosin  en- 
tirely white.     Since  cream  or  whiting  may  be 


16-t  THEORY  OF  EVOLUTION 

carried  by  red  eyed  flies  without  their  presence 
being  seen  until  eosin  is  used,  the  experimenter 
must  be  continually  on  the  lookout  for  such 
factors  which  may  lead  to  erroneous  conclusions 


Red 


ccLcreamJ  ee  ww  [whiting] 


eecc  eeww 

Fig.  82.  Scheme  to  indicate  influence  of  the  modifying  fac- 
tors, cream  and  whiting.  Neither  produces  any  effect  alone  but 
they  modify   other   eye  colors   such  as   eosin. 

unless  detected.  As  yet  breeders  have  not 
realized  the  important  role  that  modifiers 
have  played  in  their  results,  but  there  are  indi- 
cations at  least  that  the  heaping  up  of  modify- 
ing factors  has  been  one  of  the  ways  in  which 


THEORY  OF  EVOLUTION  165 

highly  specialized  domesticated  animals  have 
been  produced.  Selection  has  accomplished 
this  result  not  by  changing  factors,  but  by 
picking  up  modifying  factors.  The  demonstra- 
tion of  the  presence  of  these  factors  has  already 
been  made  in  some  cases.  Their  study  prom- 
ises to  be  one  of  the  most  instructive  fields  for 
further  work  bearing  on  the  selection  hypo- 
thesis. 

In  addition  to  these  well  recognized  methods 
by  which  artificial  selection  has  produced  new 
things  we  come  now  to  a  question  that  is  the 
very  crux  of  the  selection  theory  today.  Our 
whole  conception  of  selection  turns  on  the  an- 
swer that  we  give  to  this  matter  and  if  I  appear 
insistent  and  go  into  some  detail  it  is  because  I 
think  that  the  matter  is  worth  very  careful 
consideration. 

Ake  Factors  Changed  Through 
Selection  '. 

As  we  have  seen,  the  variation  that  we  find 
from  individual  to  individual  is  due  in  part 
to  the  environment;  this  can  generally  be 
demonstrated.      Other    differences    in    an    or- 


166  THEORY  OF  EVOLUTION 

dinary  population  are  recognized  as  due  to  dif- 
ferent genetic  (hereditary)  combinations. 
Xo  one  will  dispute  this  statement.  But  is  all 
the  variability  accounted  for  in  these  two  ways  ? 
May  not  a  factor  itself  fluctuate?  Is  it  not 
a  priori  probable  that  factors  do  fluctuate? 
Why,  in  a  word,  should  we  regard  factors  as  in- 
violate when  we  see  that  everything  else  in 
organisms  is  more  or  less  in  amount  ?  I  do  not 
know  of  any  a  priori  reason  why  a  factor  may 
not  fluctuate,  unless  it  is,  as  I  like  to  think,  a 
chemical  molecule.  We  are,  however,  dealing 
here  not  with  generalities  but  with  evidence, 
and  there  are  three  known  methods  by  means 
of  which  it  has  been  shown  that  variability, 
other  than  environmental  or  recombinational, 
is  not  due  to  variability  in  a  factor,  nor  to  vari- 
ous "potencies"  possessed  by  the  same  factors. 

( 1 )  By  making  the  stock  uniform  for  all  of 
its  factors — chief  factors  and  modifiers  alike. 
Any  change  in  such  a  stock  produced  by  selec- 
tion would  then  be  due  to  a  change  in  one  or 
more  of  the  factors  themselves.  Johannsen's 
experiment  is  an  example  of  this  sort. 

(2)  The  second  method  is  one  that  is  capa- 


THEORY  OF  EVOLUTION 


ur, 


ble  of  demonstrating  that  the  effects  of  selec- 
tion are  actually  due  to  modifiers.     It  has  been 


Fig.  s.}  a.     Drosophila  ampelophila  with  truncate  wings. 

worked  out  in  our  laboratory,  chiefly  by  Mid- 
ler, and  used  in  a  particular  case  to  demonstrate 
that  selection  produced  its  effect  by  isolating 
modifying  factors.  For  example,  a  mutant 
type  called  truncate  appeared,  characterized 
by  shorter  wings,  usually  square  at  the  end, 
(fig.  83a).  The  wings  varied  from  those  of 
normal  length  to  wings  much  shorter  (fig. 
83b).     For  three  years  the  mutant  stock  was 


168  THEORY  OF  EVOLUTION 


Fig.  83  b.     Series  of  wings  of  different  length  shown  by  trun- 
cate stock  of  D.  ampelophila. 

bred  from  individuals  having  the  shorter 
wings  until  at  last  a  stock  was  obtained  in 
which  some  of  the  individuals  had  wings  much 
shorter  than  the  body.  By  means  of  linkage 
experiments  it  was  shown  that  at  least  three 
factors  were  present  that  modified  the  wings. 
These  were  isolated  by  means  of  their  linkage 
relations,  and  their  mutual  influence  on  the  pro- 
duction of  truncate  wings  was  shown. 


THEORY  OF  EVOLUTION  169 

An  experiment  of  this  kind  can  only  be  car- 
ried out  in  a  case  where  the  groups  of  linked 
gens  are  known.  At  present  Drosophila  is 
the  only  animal  (or  plant)  sufficiently  well 
known  to  make  this  test  possible,  but  this  does 
not  prove  that  the  method  is  of  no  value.  On 
the  contrary  it  shows  that  any  claim  that  factors 
can  themselves  be  changed  can  have  no  finality 
until  the  claim  can  be  tested  out  by  means  of 
the  linkage  test.  For  instance,  bar  eye  (fig. 
31)  arose  as  a  mutation.  All  our  stock  has  de- 
scended from  a  single  original  mutant.  But 
Zeleny  has  shown  that  selection  within  our 
stock  will  make  the  bar  eye  narrower  or 
broader  according  to  the  direction  of  selection. 
It  remains  to  be  shown  in  this  case  how  selec- 
tion has  produced  its  effects,  and  this  can  be 
done  by  utilizing  the  same  process  that  was 
used  in  the  case  of  truncate. 

Another  mutant  stock  called  beaded  (fig. 
84),  has  been  bred  for  five  years  and  selected 
for  wings  showing  more  beading.  In  extreme 
cases  the  wings  have  been  reduced  to  mere 
stumps  (see  stumpy,  fig.  .5),  but  the  stock 
shows  great  variability.     It  is  probable  here 


170 


THEORY  OF  EVOLUTION 


as  Dexter  has  shown,  that  a  number  of  mutant 
factors  that  act  as  modifiers  have  been  picked 


Fig.  84.     Two  flies  showing  beaded  win  gs. 

up  in  the  course  of  the  selection,  and  when  it 
is  recalled  that  during  those  five  years  over 
125  new  characters  have  appeared  elsewhere 
it  does  not  seem  improbable  that  factors  also 
have  appeared  that  modify  the  wings  of  this 
stock. 

(3)  The  third  method  is  one  that  has  been 
developed  principally  by  East  for  plants;  also 
by    MacDowell    for   rabbits    and    flies.      The 


THEORY  OF  EVOLUTION 


171 


method  does  not  claim  to  prove  that  modifiers 
are  present,  but  it  shows  why  certain  results 
are  in  harmony  with  that  expectation  and  can 
not  be  accounted  for  on  the  basis  that  a  factor 
has  changed.  Let  me  give  an  example.  When 
a  Belgian  hare  with  large  body  was  crossed  to 
a  common  rabbit  with  a  small  body  the  hybrid 
was  intermediate  in  size.  When  the  hybrid 
was  crossed  back  to  the  smaller  type  it  pro- 
duced rabbits  of  various  sizes  in  apparently 
a  continuous  series.  MacDowell  made  meas- 
urements of  the  range  of  variability  in  the  first 
and  in  the  second  generations. 

He  found  that  the  variability  was  smaller  in 
the  first  generation  than  in  the  second  genera- 

ClassificaHon  in  relation  to  parents  based  on  skull  length*  and  tiln  i 
lengths,  to  show  the  relative  variability  of  two  measurements  an  ~ 
of  the  first  generation  (F\)  aocl  flip  hack  cross  (B.  C.) 


CHARACTER       GENE-     _13_12 


Length  of  (  Fi 

skull       i  B.C. 

Lensrthof  (  F, 

ulna       i  B.C. 


•11  -10  -li  -S  -7 


2  3  1 


3 

4   4 


1842 


32 


120261719 


same  table  continued 


CHARACTER       GENE- 
RATION 


Length  of  ( 
skull       1 

Length  of  j 
ulna        I 


F 

B(J.  393416 

F,  111 

BX!.  'l*  1512 


91(1111213 


old 
s  -1 


13lloll 


1516 


7  3   2  2 

3  1 

3  17  3 

5  2    4  2 


18  19 


2021222324 


25 


172  THEORY  OF  EVOLUTION 

tion  (back  cross).  This  is  what  is  expected  if 
several  factor-differences  were  involved,  be- 
cause the  hybrids  of  the  first  generation  are 
expected  to  be  more  uniform  in  factorial  com- 
position than  are  those  in  the  second  generation 
which  are  produced  by  recombination  of  the 
factors  introduced  through  their  grandparents. 
Excellent  illustrations  of  the  same  kinds  of 
results  have  been  found  in  Indian  corn.  As 
shown  in  figure  85  the  length  of  the  cob  in  Fx 
is  intermediate  between  the  parent  types  while 
in  F2  the  range  is  wider  and  both  of  the  original 
types  are  recovered.  East  states  that  similar 
relations  have  been  found  for  18  characters 
in  corn.  Emerson  has  recently  furnished 
further  illustrations  of  the  same  relations  in 
the  length  of  stalks  in  beans. 

A  similar  case  is  shown  by  a  cross  between 
fantail  and  common  pigeons  (fig.  86).  The 
latter  have  twelve  feathers  in  the  tail,  while  the 
selected  race  from  which  the  fantails  came  had 
between  28  and  38  feathers  in  the  tail.  The  F1 
offspring  (forty-one  individuals)  showed  (fig. 
87)  between  12  and  20  tail  feathers,  while 
in   F2   the   numbers   varied    between    12    and 


THEORY  OF  EVOLUTION 


173 


Fit..  8.5.  Cross  between  two  races  of  Indian  corn,  one  with 
short  cobs  and  one  with  long  cobs.  The  range  of  variability  in 
F1  is  less  than  that  in   ¥,.     (After  East.) 


Fig.  8(j.     Cross  of  pigeon  with  normal  tail  P,  and  fantail  Pt; 
F,    bird   below. 


THEORY  OF  EVOLUTION 


175 


25.  Here  one  of  the  grand-parental  tv^es 
reappears  in  large  numbers,  while  the  extreme 
of  the  other  grand-parental  type  did  not  re- 
appear (in  the  counts  obtained),  although  the 
F2  number  would  probably  overlap  the  lower 


F< 


I 
I  I 
-I 1_ 


Fig.  87.  Cross  of  normal  and  fantail  pigeons.  (See  Fig. 
86.)  The  F2  range  is  wider  than  that  of  F,.  The  normal  grand- 
parental  type  of  12  feathers  was  recovered  in  ¥.,  but  the  higher 
numbers  characteristic  of  fantails  were  not  recovered. 


176 


THEORY  OF  EVOLUTION 


limits  of  the  race  of  fantail  grandparents  had 
not  a  selected  (surviving)  lot  been  taken  for 
the  figures  given  in  the  table. 

The  preceding  account  attempts  to  point  out 
how  I  should  prefer  to  interpret  the  problem 
of  selection  in  the  light  of  the  most  recent  work 
on  breeding.  But  I  would  give  a  very  incom- 
plete account  of  the  whole  situation  if  I  neg- 
lected to  include  some  important  work  which 
has  led  some  of  my  fellow-workers  to  a  very 
different  conclusion. 

Castle  in  particular  is  the  champion  of  a 
view  based  on  his  results  with  hooded  rats. 


Fig.    88.      Scheme    to   show   classes    of    hooded    rats    used   by 
Castle.      (After  Castle.) 


Starting  with  individuals  which  have  a  narrow 
black  stripe  down  the  back  he  selected  for  a 
narrower   stripe   in   one   direction   and   for   a 


THEORY  OF  EVOLUTION  ITT 

broader  stripe  in  the  other.  As  the  diagram 
shows  (fig.  88)  Castle  has  succeeded  in  pro- 
ducing in  one  direction  a  race  in  which  the 
dorsal  stripe  has  disappeared  and  in  the  other 
direction  a  race  in  which  the  black  has  extended 
over  the  back  and  sides,  leaving  only  a  white 
mark  on  the  belly.  Neither  of  these  extremes 
occurs,  he  believes,  in  the  ordinary  hooded  race 
of  domesticated  rats.  In  other  words  no  mat- 
ter how  many  of  them  came  under  observa- 
tion the  extreme  types  of  his  experiment  would 
not  be  found. 

Castle  claims  that  the  factor  for  hoodedness 
must  be  a  single  Mendelian  unit,  because  if 
hooded  rats  are  crossed  to  wild  gray  rats  with 
uniform  coat  and  their  offspring  are  inbred 
there  are  produced  in  F2  three  uniform  rats  to 
one  hooded  rat.  Castle  advances  the  hypothe- 
sis that  factors — by  which  he  means  Mendelian 
factors — may  themselves  vary  in  much  the 
same  way  as  do  the  characters  that  they  stand 
for.  He  argues,  in  so  many  words,  that  since 
we  judge  a  factor  by  the  kind  of  character  it 
produces,  when  the  character  varies  the  factor 
that  stands  for  it  may  have  changed. 


178  THEORY  OF  EVOLUTION 

As  early  as  1903  Cuenot  had  carried  out 
experiments  with  spotted  mice  similar  to  those 
of  Castle  with  rats.  Cuenot  found  that  spotted 
crossed  to  uniform  coat  color  gave  in  F2  a  ratio 
of  three  uniform  to  one  sj)otted,  yet  selection 
of  those  spotted  mice  with  more  white  in  their 
coat  produced  mice  in  successive  generations 
that  had  more  and  more  white.  Conversely 
Cuenot  showed  that  selection  of  those  spotted 
mice  that  had  more  color  in  their  coat  produced 
mice  with  more  and  more  color  and  less  white. 
Cuenot  does  not  however  bring  up  in  this  con- 
nection the  question  as  to  how  selection  in  these 
spotted  mice  brings  about  its  results. 

Without  attempting  to  discuss  these  results 
at  the  length  that  they  deserve  let  me  briefly 
state  why  I  think  Castle's  evidence  fails  to  es- 
tablish his  conclusion. 

In  the  first  place  one  of  the  premises  may  be 
wrong.  The  three  to  one  ratio  in  F2  by  no 
means  proves  that  all  conditions  of  hoodedness 
are  due  to  one  factor.  The  result  shows  at  most 
that  one  factor  that  gives  the  hooded  types  is 
a  simple  Mendelian  factor.  The  changes  in 
this  type  may  be  caused  by  modifying  factors 


THEORY  OF  EVOLUTION  179 

that  can  show  an  effect  only  when  hoodedness 
is  itself  present.  That  this  is  not  an  imaginary 
objection  but  a  real  one  is  shown  by  an  experi- 
ment that  Castle  himself  made  which  furnishes 
the  ground  for  the  second  objection. 

Second.  If  the  factor  has  really  changed  its 
potency,  then  if  a  very  dark  individual  from 
one  end  of  the  series  is  crossed  to  a  wild  rat  and 
the  second  generation  raised  we  should  expect 
that  the  hooded  F2  rats  would  all  be  dark  like 
their  dark  grandparent.  When  Castle  made 
this  test  he  found  that  there  were  many  grades 
of  hooded  rats  in  the  F2  progeny.  They  were 
darker,  it  is  true,  as  a  group  than  were  the 
original  hooded  group  at  the  beginning  of  the 
selection  experiment,  but  they  gave  many  in- 
termediate grades.  Castle  attempts  to  explain 
this  by  the  assumption  that  the  factor  made 
pure  by  selection  became  contaminated  by  its 
normal  allelomorph  in  the  Fx  parent,  but  not 
only  does  this  assumption  appear  to  beg  the 
whole  question,  but  it  is  in  flat  contradiction 
with  what  we  have  observed  in  hundreds  of 
Mendelian  cases  where  no  evidence  for  such 
a  contamination  exists. 


180  THEORY  OF  EVOLUTION 

Later  Castle  crossed  some  of  the  extracted 
rats  of  average  grade  (3.01)  from  the  plus 
series  to  the  same  wild  race  and  got  F2  hooded 
rats  from  this  cross.  These  F2  hooded  rats  did 
not  further  approach  the  ordinary  range  but 
were  nearer  the  extreme  selected  plus  hooded 
rats  (3.38)  than  were  the  F2's  extracted  from 
the  first  cross  (2. .59).  Castle  concludes  from 
this  that  multiple  factors  can  not  account  for 
the  result.  As  a  matter  of  fact,  Castle's  evi- 
dence as  published  does  not  establish  his  con- 
clusion because  the  wild  rats  used  in  the  second 
experiment  may  have  carried  plus  modifiers. 
This  could  only  be  determined  by  suitable  tests 
which  Castle  does  not  furnish.  This  is  the 
crucial  point,  without  which  the  evidence  carries 
no  conviction. 

Furthermore,  from  Castle's  point  of  view, 
these  latest  results  would  seem  to  increase  the 
difficulty  of  interpretation  of  his  first  F2  ex- 
tracted cross,  and  it  is  now  the  first  result  that 
calls  for  explanation  if  one  accepts  his  later 
conclusion. 

These  and  other  objections  that  might  be 
taken  up  show,  I  think,  that  Castle's  experi- 


THEORY  OF  EVOLUTION  181 

ment  with  hooded  rats  fails  entirely  to  estab- 
lish his  contention  of  change  in  potency  of  the 
germ  or  of  contamination  of  factors,  while  on 
the  contrary  they  are  in  entire  accord  with  the 
view  that  he  is  dealing  with  a  case  of  modify- 
ing factors. 

Equally  important  are  the  results  that  Jen- 
nings has  obtained  with  certain  protozoa.  Par- 
amecium multiplies  by  dividing  across  in  the 


000H<H« 


00903JM» 


0000003M9 


Fig.  89.     Races  of  Paramecium.      (After  Jennings.) 


182  THEORY  OF  EVOLUTION 

middle,  each  half  replacing  its  lacking  part. 
Both  the  small  nucleus  (micronucleus)  and  the 
large  nucleus  (macronucleus)  divide  at  each 
division  of  the  body.  Jennings  found  that 
while  individuals  descended  from  a  single 
Paramecium  vary  in  size  ( fig.  89 ) ,  yet  the 
population  from  a  large  individual  is  the  same 
as  the  population  derived  from  a  small  individ- 
ual. In  other  words,  selection  produces  no  re- 
sult and  the  probable  explanation  is,  of  course, 
that  the  different  sizes  of  individuals  are  due 
to  the  environment,  while  the  constancy  of  the 
type  is  genetic.  Jennings  found  a  number  of 
races  of  paramecium  of  different  sizes  living 
under  natural  conditions.  The  largest  indi- 
vidual of  a  small  race  might  overlap  the  small- 
est individual  of  other  larger  races  (fig.  89)  ; 
nevertheless  each  kind  reproduced  its  particu- 
lar race.  The  results  are  like  those  of  Johann- 
sen  in  a  general  way,  but  differ  in  that  repro- 
duction takes  jnace  in  paramecium  by  direct 
division  instead  of  through  self-fertilization  as 
in  beans,  and  also  in  that  the  paramecia  were 
probably  not  homozygous.  Since,  however, 
so  far  as  known  no  "reduction"  takes  place  in 


THEORY  OF  EVOLUTION 


183 


Paramecium  at  each  division,  the  genetic  com- 
position of  parent  and  offspring  shonld  be  the 
same.  Whether  pseudo-parthenogenesis  that 
Woodruff  and  Erdmann  have  found  occurring 
in  paramecium  at  intervals  involves  a  redistri- 
bution of  the  hereditary  factors  is  not  clear. 
Jennings's  evidence  seems  incompatible  with 
such  a  view. 

More  recently  one  of  Jennings's  students, 
Middleton,  has  made  a  careful  series  of  selec- 
tion experiments  with  Stylonychia  [fig.  90)  in 
which  lie  selected  for  lines  showing  more  rapid 


Fig.    90.      Stylonychia    showing    division    into    two.      (After 
Stein.) 


184  THEORY  OF  EVOLUTION 

or  slower  rates  of  division.  His  observations 
seem  to  show  that  his  selection  separated  two 
such  lines  that  came  from  the  same  original 
stock.  The  rapidity  of  the  effects  of  selection 
seems  to  preclude  the  explanation  that  pseudo- 
parthenogenesis  has  complicated  the  results. 
Nevertheless,  the  results  are  of  such  a  kind  as 
to  suggest  that  they  were  due  to  selection  of 
vegetative  (somatic)  differences  and  that  no 
genetic  change  of  factors  was  involved,  for  his 
conclusion  that  the  rapidity  with  which  the 
effects  gained  by  long  selection  might  be  sud- 
denly reversed  when  selection  was  reversed  is 
hardly  consistent  with  an  interpretation  of  the 
results  based  on  changes  in  the  "potencies"  of 
the  factors  present. 

Equally  striking  are  the  interesting  experi- 
ments that  Jennings  has  recently  carried  out 
with  Difflugia  (fig.  91).  This  protozoon  se- 
cretes a  shell  about  itself  which  has  a  charac- 
teristic shape,  and  often  carries  spines.  The 
opening  at  one  end  of  the  shell  through  which 
the  protoplasm  protrudes  to  make  the  pseudo- 
podia  is  surrounded  by  a  rim  having  a  charac- 
teristic   pattern.      The    protoplasm    contains 


THEORY  OF  EVOLUTION  185 

several  nuclei  and  in  addition  there  is  scattered 
material  or  particles  called  chromidia  that  are 


Fig.  91.     Difflugia   Corona.      (After  Cash.) 

supposed  to  be  chromatic  in  nature  and  related 
to  the  material  of  the  nuclei,  possibly  by  direct 
interchange. 

When  Difflugia  divides,  part  of  the  proto- 
plasm protrudes  from  the  opening  and  a  new 
shell  is  secreted  about  this  mass  which  becomes 
a  daughter  individual.  The  behavior  of  the 
nucleus  and  of  the  chromidia  at  this  time  is  ob- 
scure, but  there  is  some  evidence  that  their 
materials  may  be   irregularly   distributed   be- 


186 


THEORY  OF  EVOLUTION 


tween  parent  and  offspring.  If  this  is  correct, 
and  if  in  the  protozoa  the  chromatin  has  the 
same  influence  that  it  seems  to  have  in  higher 
animals,  the  mode  of  reproduction  in  Difflugia 
would  be  expected  to  give  little  more  than  ran- 
dom sampling  of  the  germ  plasm. 

Jennings  was  able  by  means  of  selection  to 
get  from  the  descendants  of  one  original  indi- 


Fig.  92.     Races  of  Difflugia.     (After  Leidy.) 


vidual  a  number  of  different  types  that  them- 
selves bred  true,  except  in  so  far  as  selection 
could  affect  another  change  in  them.  In  this 
connection  it  is  interesting  to  note  that  Leidy 


THEORY  OF  EVOLUTION  187 

has  published  figures  of  Difflugia  (fig.  92)  that 
show  that  a  great  many  "types"  exist.  If 
through  sexual  union  (a  process  that  occurs  in 
Difflugia)  the  germ  plasm  (chromatin)  of  these 
wild  types  has  in  times  past  heen  recombined, 
then  selection  would  be  expected  to  separate 
certain  types  again,  if,  at  division,  irregular 
sampling  of  the  germ  plasm  takes  place.  Until 
these  jDoints  are  settled  the  bearing  of  these 
important  experiments  of  Jennings  on  the 
general  problem  of  selection  is  uncertain. 

How  Does  Natural  Selection  Influence 
the  Course  of  Evolution? 

The  question  still  remains:  Does  selection 
play  any  role  in  evolution,  and,  if  so,  in  what 
sense  \  Does  the  elimination  of  the  unfit  influ- 
ence the  course  of  evolution,  except  in  the  nega- 
tive sense  of  leaving  more  room  for  the  fit? 
There  is  something  further  to  be  said  in  this 
connection,  although  opinions  may  differ  as  to 
whether  the  following  interpretation  of  the 
term  "natural  selection"  is  the  only  possible 
one. 

If  through  a  mutation  a  character  appears 


188 


THEORY  OF  EVOLUTION 


that  is  neither  advantageous  nor  disadvanta- 
geous, but  indifferent,  the  chance  that  it  may 
become  established  in  the  race  is  extremely 
small,  although  by  good  luck  such  a  thing  may 
occur  rarely.  It  makes  no  difference  whether 
the  character  in  question  is  a  dominant  or  a 


Recent 
Pleuloccne- 
Upper  Pliocene 


CLE  PHAS 
(abort  ch'n) 


Lower  Pliocene      TETRA8EL0D0N 

[lOWCiRCSTRIS  ST*C£| 

Upper  Miocene        {shortening  chinf 


Middle  Miocene    TETRABEL030N 

Iancustidens  STACl) 
Lifer  Miocene  (longchinj 


,, „,  Migration  from  Africa 

UmrrOitiiocene  J  r  MJ 

'I  "  iiUk  tumpr  -  Asia 


j     (I, 


lemer  Otigocene\ 

PALAEOMASTODON 

VpperEoctne    \    ('"•^""-P  <*") 


MidoieEoeene 
Lmner  Eeeene 


MOCRITHERIUM 


•  m 


Fig.  93.     Evolution   of  elephant's   skulls.      (After  Dendv.) 


THEORY  OF  EVOLUTION  189 

recessive  one,  the  chance  of  its  becoming  estab- 
lished is  exactly  the  same.  If  through  a  muta- 
tion a  character  appears  that  has  an  injurious 
effect,  however  slight  this  may  be,  it  has  prac- 
tically no  chance  of  becoming  established. 
If  through  a  mutation  a  character  appears 


Fig.  94.     Evolution  of  elephant's  trunk.   (After  Lull.) 

that  has  a  beneficial  influence  on  the  individual, 
the  chance  that  the  individual  will  survive  is 
increased,  not  only  for  itself,  but  for  all  of  its 


190  THEORY  OF  EVOLUTION 

descendants  that  come  to  inherit  this  character. 
It  is  this  increase  in  the  number  of  individuals 
possessing  a  particular  character,  that  might 
have  an  influence  on  the  course  of  evolution. 
This  gives  a  better  chance  for  improvement  by 
several  successive  steps;  but  not  because  the 
species  is  more  likely  to  mutate  again  in  the 
same  direction.  An  imaginary  example  will 
illustrate  how  this  happens:  When  elephants 
had  trunks  less  than  a  foot  long,  the  chance  of 
getting  trunks  more  than  one  foot  long  was 
in  proportion  to  the  length  of  trunks  already 
present  and  to  the  number  of  individuals;  but 
increment  in  trunk  length  is  no  more  likely  to 
occur  from  an  animal  having  a  trunk  more  than 
one  foot  long  than  from  an  animal  with  a 
shorter  trunk. 

The  case  is  analogous  to  tossing  pennies.  At 
any  stage  in  the  game  the  chance  of  accumu- 
lating a  hundred  heads  is  in  proportion  to  the 
number  of  heads  already  obtained,  and  to  the 
number  of  throws  still  to  be  made.  But  the 
number  of  heads  obtained  has  no  influence  on 
the  number  of  heads  that  will  appear  in  the  next 
throw. 


Fig.  9.5.  Evolution  of  elephant's  trunk:  above  Maeritherium, 
in  the  middle  Tetrabelodon  (After  Lancaster);  below  African 
elephants   (After  Gambier   Bolton). 


192  THEORY  OF  EVOLUTION 

Owing  then  to  this  property  of  the  germ 
plasm  to  duplicate  itself  in  a  large  number  of 
samples  not  only  is  an  opportunity  furnished  to 
an  advantageous  variation  to  become  exten- 
sively multiplied,  but  the  presence  of  a  large 
number  of  individuals  of  a  given  sort  preju- 
dices the  probable  future  result. 

The  question  may  be  raised  as  to  whether  it 
is  desirable  to  call  selection  a  creative  process. 
There  are  so  many  supernatural  and  mystical 
implications  that  hang  around  the  term  creative 
that  one  can  not  be  too  careful  in  stating  in 
what  sense  the  term  is  to  be  used.  If  by  crea- 
tive is  meant  that  something  is  made  out  of 
nothing,  then  of  course  there  is  no  need  for  the 
scientist  to  try  to  answer  such  a  question.  But 
if  by  a  creative  process  is  meant  that  something 
is  made  out  of  something  else,  then  there  are 
two  alternatives  to  be  reckoned  with. 

First,  if  it  were  true  that  selection  of  an  indi- 
vidual of  a  certain  kind  determines  that  new 
variations  in  the  same  direction  occur  as  a  con- 
sequence of  the  selection,  then  selection  would 
certainly  be  creative.  How  this  could  occur 
might  be  quite  unintelligible,  but  of  course  it 


THEORY  OF  EVOLUTION  193 

might  be  claimed  that  the  point  is  not  whether 
we  can  explain  how  creation  takes  place,  but 
whether  we  can  get  verifiable  evidence  that  such 
a  kind  of  thing  happens.  This  possibility  is 
disposed  of  by  the  fact  that  there  is  no  evidence 
that  selection  determines  the  direction  in  which 
variation  occurs. 

Second,  if  you  mean  by  a  creative  process 
that  by  picking  out  a  certain  kind  of  individual 
and  multiplying  its  numbers  a  better  chance  is 
furnished  that  a  certain  end  result  will  be  ob- 
tained, such  a  process  may  be  said  to  be  crea- 
tive. This  is,  I  think,  the  proper  use  of  the 
term  creative  in  a  mechanistic  sense. 

Conclusions 

In  reviewing  the  evidence  relating  to  selec- 
tion I  have  tried  to  handle  the  problem  as  ob- 
jectively as  I  could. 

The  evidence  shows  clearly  that  the  charac- 
ters of  wild  animals  and  plants,  as  well  as  those 
of  domesticated  races,  are  inherited  both  in  the 
wild  and  in  the  domesticated  forms  according 
to  Mendel's  Law. 

The  causes  of  the  mutations  that  give  rise 


194  THEORY  OF  EVOLUTION 

to  new  characters  we  do  not  know,  although 
we  have  no  reason  for  supposing  that  they  are 
due  to  other  than  natural  processes. 

Evolution  has  taken  place  by  the  incorpora- 
tion into  the  race  of  those  mutations  that  are 
beneficial  to  the  life  and  reproduction  of  the 
organism.  Natural  selection  as  here  defined 
means  both  the  increase  in  the  number  of 
individuals  that  results  after  a  beneficial  muta- 
tion has  occurred  ( owing  to  the  ability  of  living 
matter  to  propagate)  and  also  that  this  prepon- 
derance of  certain  kinds  of  individuals  in  a 
population  makes  some  further  results  more 
probable  than  others.  More  than  this,  natural 
selection  can  not  mean,  if  factors  are  fixed  and 
are  not  changed  by  selection. 


INDEX 


Abnormal  abdomen   109 

Abraxas  78-81 

Allantois    17 

Allelomorphs  83-84 

Altenburg   112 

Amnion  1  (J-l  7 

Andalusian   fowl  4.5,  4(i 

Annelids  22 
Antlered  wing-  111 
Apterous  wing  11 
Arc   wing-   111 
Aristae    104 

Bar  eve  67,  108,  169 
Bateson  18,  34,  36 

Beaded  wing  11,  115 

Beans   147-149,   157 

Belgian  hare   171 

Bent   wing   116 

Bergson  30,  31 

Bildungstrieb  34 

Biogenetic  law  15,  18,  19,  2\ 

Biometricians   156 

Bird  21,  23 

Bithorax  65,  113,  113, 

Black  body  color  111,  133 

Blakeslee    152 

Bridges    114,    143,   163 

British   Association  36 

Briinn    10 

Buff  eve   color    109 

B u ton' 21 

Castle    176-180 

Cat   33 

Cell  90,  91 

Chance  variations  37 

Chick    Hi,    17,   20 

Chromatin    lsi 


Chromosome  group  of  Drosoph- 

ila   102 
Chromosomes  91,  95,  96,  98,  130, 

131,   133 
Cleavage  21,  22,  94 
Clover  butterfly  62 
Club  wing  69,  70,  108 
Colias  philodice  62 
Color  blindness  77,  125 
Comb  of  Drosophila   103 
Combs  of  fowls  33,  54 
Comparative    anatomy    7,    8,    9. 

14 
Corn  150,  153,  112 
Correns  41 
Cosmogonies  27 
Cream  eye  color  163,  164 
Crepidula  22 

Criss-cross    inheritance    7S 
Crossing  over   131-133 
Cuenot  178 
Curled  wing  115 
Curved  wing  111 
Curve  of  probability  149 
Cut    wing    11,    104 

Hacks   legs    112 

Dahlgren  62 

Darwin  15,  24,  38,  32,  36-o7,  64, 

145,  146,  152,  154-156 
Dendy   188 

De   Vries   18,  147,   156 
Dexter  170 
Dichaete   114 
Difflugia   184-187 
Discontinuous  variation   13 
Disuse  31 
Drosophila    ampelophila    10,    12, 


id: 


196 


INDEX 


13,   48-50,   60,   75,   84,   85,  93,     Hoge  66 

100,  103,   119,   155,  16:2,  169     Horse,  evolution  of,  6 
Drosophila  repleta  76 

Duplication  of  legs  109  Indian  corn  172,  173 

Dwarf  114  Interference  137,  138 


East   170,   172 

Ebony  50,  55,  56,   115 

Egg  91,  94 

Elephant    191 

Elephants'  skulls  188 

Elephants'  trunks   190 

Embryology  13-23 

Emerson  172 

Environment  27 

Eosin  eve  color  61,  107,  163 

Erdmann  183 

Evolution  Creatrice  30 

Evolution — three  kinds  of — 

1,  2,  4 
Eye   color    13 
Eyeless  66,  115 

Factorial  theory  89 
Factors  of  Drosophila  143 
Fantails  172,  175 
Fertilization   91 
Fish  16,  20,  21 
Flatworms  22 
Fluctuations  12 
Forked  bristles  106 
Fowl  77 
Fused  veins  107,   108 

Galton  154 

Geneticist  26 

Germ-plasm  142 

Geoffrov  St.  Hiliare  27 

Giant   1*14 

Gill-slits  20,  21,  23 

Groups  I,  II,  III,  IV,  100-1  IS 

Haeckel  15 

Haemophilia    77 
Heliotropism    106,   107 
Himalyan   rabbits  83 

History    1%,   6 


Janssens  132 
Jauntv  wing  111 
Jennings  161,  181-184,  186 
Johannsen     156,     157,     159-161, 
166,   182 

Lamarck   31-34 

Langshan  77 

Leaves   147 

Leidy   186 

Lethal   105 

Linkage  groups   103 

Lizard    23 

Localization    of   factors    118 

MacDowell  155,  170,  171 
Maeritherium  191 
Mammal  16,  21,  23 
Man  20,  77,  125,  126 
Map  of  Chromosomes   136 
Maroon  eye  color  114 
Mendel  40,  41,  52,  89 
Mendelian  heredity  39 
Mendel's  law  41-59,  64,  124 
Mendel's  second  law  52 
Mesenchyme    cells    22 
Mesoderm   cells  22 
Metaphysician  30 
Mice  33,   178 
Middleton   183 
Miniature  wing  108 
Mirabilis  12 

Modifiers  163,  164,  170,  171 
Molluscs  22 
Mouse   83 
Midler   112,  167 
Mutations   35,  39,   84 

Xageli  34,  35 

Natural   Selection   36,    145,   146, 
187-194 


INDEX 


197 


Nisus   formativus  31< 
Non-disjunction    139-142 
Notch   wing   104-100 
Nucleus    91 

Origin  of  Species  35,   14.5 
Orthogenesis  34 

Paleontology    24-97 
Papilio   polytes   63 
Papilio  turnus  63 
Paramecium   181,   182 
Paratettix  81 
Peach  eye  color  114 
Pea  comb  54 
Pearl   161 
Peas   47 

Pigeons  172,  174,  175 
Pink  eye  color   114,   1 1.3 
Plana rian  22 
Plymouth   Rock  77 
Podarke  22 
Polar   bodies    126 
Pole  arms  5 
Protozoa  181 

Pseudo-parthenogenesis    183 
Purple  eye  color   109 
Purpose    4 

Rabbits   83,   170 
Rats   176-180 
Reduction   division    182 
Reproductive   cells   96 
Ruby    eye   color    106 
Rudimentary   organ    116 
Rudimentary   wing   70,   71,   107 

Sable    body    color    107 
Science,   definition    of,   6 
Segregation  41 
Selenka  94 

Sepia  eye  color  13,  114 
Sex  chromosomes   1  is 
Sex  linked  inheritance  75, 

118-130 
Sexual  dimorphism  62 
Slice))  :VA 


Single   comb   54 

Sooty  body  color  50,  111,  115 

Speck    68,"  69,    111 

Spencer    145 

Spermatozoon    91,    98 

Stars,  evolution   of,  6 

St.   Hiliare  27-30 

Strap  wing  110,  111 

Stumpy  wing   11 

Sturtevant  76,   143 

Stylonychia   183 

Survival  of  the  fittest   146 

Systematist    85 

Tails  :i'A 

Tan  thes  106,  107 
Tetrabelodon  191 
Trefoil   111 

Truncate    wing    111,     11;?,    167, 
168 

Unfolding  principle  34 
I  nio  22 

Unit   character  74,  75 
Use  31 

Variation,  discontinuous  13 
Vermilion  eye  color  108,  163 
Vestigial   wing   11,  oo,  56,   109, 

133 
Vital  force  34 

Wallace  36 

Walnut  comb  54 

Weismann   17,  31-33 

Wilson,  E.  B.  125 

Wingless   67 

Winiwarter    126 

White  eye  color  13,  75,  119-130 

Whiting  eve  color  163,  164 

Woodruff '  183 

Yellow   bodv  color   108,  133 
Yolk  sac  16,  17 

Zelenv    169 


